Alternating bias hot carrier solar cells

ABSTRACT

Designs of extremely high efficiency solar cells are described. A novel alternating bias scheme enhances the photovoltaic power extraction capability above the cell band-gap by enabling the extraction of hot carriers. When applied in conventional solar cells, this alternating bias scheme has the potential of more than doubling their yielded net efficiency. When applied in conjunction with solar cells incorporating quantum wells (QWs) or quantum dots (QDs) based solar cells, the described alternating bias scheme has the potential of extending such solar cell power extraction coverage, possibly across the entire solar spectrum, thus enabling unprecedented solar power extraction efficiency. Within such cells, a novel alternating bias scheme extends the cell energy conversion capability above the cell material band-gap while the quantum confinement structures are used to extend the cell energy conversion capability below the cell band-gap. Light confinement cavities are incorporated into the cell structure in order to allow the absorption of the cell internal photo emission, thus further enhancing the cell efficiency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application No.14/028,344 filed Sep. 16, 2013, now U.S. Pat. No. 9,171,970, which is acontinuation of U.S. patent application No. 13/245,527 filed Sep. 26,2011, now U.S. Pat. No. 8,536,444, which is a divisional of U.S. patentapplication No. 13/165,590 filed Jun. 21, 2011, now U.S. Pat. No.8,217,258, which is a continuation-in-part of U.S. patent applicationNo. 12/833,661 filed Jul. 9, 2010, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of solar cells, solar powersystems and methods.

2. Prior Art

Solar Cell Efficiency Loss Mechanisms

Today's solar cells operate substantially below the theoreticalefficiency level established by Sockley-Queisser Model (SQ-Model [W.Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency ofp-n Junction Solar Cell”, J. App. Phys., Vol. 32, pp. 510-519, March1961]). The solar cell designs described herein can exceed the limitestablished by the SQ-Model. In order to improve the solar cellefficiency toward exceeding the SQ-Model, it is important to understandthe mechanisms that cause the degradation in solar cell efficiency. FIG.1 (adapted from “Third Generation Photovoltaics: Advanced Solar EnergyConversion”, M. A. Green, Springer, New York, 2003, pp. 35-43)illustrates these efficiency degradation mechanisms in the singlejunction solar cell. Referring to FIG. 1, the efficiency loss mechanismsin a solar cell include the effects listed below.

Represents incoming photons with energies (E_(p)) below the band-gap(labeled E_(g)) of the device that are not absorbed, thus their energyis not converted into current by the solar cell.

Represents incoming photons with energies above the band-gap which areabsorbed but lose their excess energy as heat due to the relaxation ofthe photo-excited electrons and holes (carriers) to the conduction bandminimum (CBM) and the valance band maximum (VBM); respectively, byproducing phonons (represented in FIG. 1 by the dashed lines). In thisloss mechanism, first the photo-excited carriers having energies abovethe solar cell material band-gap will equilibrate with other carriers toform a carrier population that can be described by a Boltzmandistribution. as shown in FIG. 2, wherein:

-   {circle around (1)} Thermal equilibrium before polychromatic light    pulse.-   {circle around (2)} Photo excited carrier immediately after    polychromatic pulse.-   {circle around (3)} Carrier scattering.-   {circle around (4)} Thermalization of “hot carriers”    (carrier/carrier equilibration).-   {circle around (5)} Hot carrier cooling (carrier/lattice    equilibration).-   {circle around (6)} Lattice thermalized carriers.-   {circle around (7)} Recombination of carriers.-   {circle around (8)} Return to thermal equilibrium.-   μElectrochemical potential-   E_(g) Solar cell material band-gap-   QFL Quasi-Fermi Levels caused by the distribution of photo-excited    carriers At this point the temperature defining the carrier    distribution would be above the material lattice temperature and    hence the carriers are referred to as “hot carriers”. Typically the    additional energy associated with the elevated temperature is    contained mainly by the electron due to its lower effective mass. In    a typical solar cell, the hot electrons will equilibrate with the    cell material lattice by giving off their excess energies to the    cell material lattice by producing phonons during their cooling time    τ_(c) period (see FIG. 2). These phonons then interact with other    phonons and the absorbed photon energies E_(p) in excess of the cell    material band-gap E_(g) are lost to heat and hence are not converted    into voltage by the solar cell. Depending on the carrier mobility    and crystal lattice characteristics of the cell material, the    carrier cooling time τ_(c) occurs in a timescale of a few    picoseconds to a few nanoseconds (see FIG. 2). As illustrated in    FIG. 2, by the end of the carrier cooling time τ_(c) the    photo-excited carrier distribution will coalesce to a narrow energy    distribution of electrons and holes near the edges of the conduction    and valence bands of the cell material; CBM and VBM, respectively.    This final stage of the photo-excited carrier lifetime would    typically last for a few microseconds (carrier recombination time    τ_(r)) as the photo-excited carriers systematically recombine giving    their residual gained energy to photons. In order for a conventional    solar cell to be able to convert the energy of the photo-excited    carriers to electric energy, the photo-excited carriers must be    separated and transported toward the cell contacts before they    recombine, meaning before the elapse of the carrier recombination    time τ_(r). The design parameters of conventional solar cells are    typically selected to achieve the carrier transport characteristics    needed to transport the photo-excited carriers to the cell contacts    before they recombine; i.e., before the elapse of the carrier    recombination time τ_(r). From the above discussion, the solar    photon energy given to the photo-excited carriers would be    dissipated in two main stages; namely carrier cooling and carrier    recombination. During the former of these two main stages; namely    the cooling stage, the photo-excited carriers give their energy in    excess of the material band-gap energy separation to phonons while    during the latter stage; namely the recombination stage, the    photo-excited carriers give their residual energy, which is    typically equal to the material band-gap energy separation, to    photons through radiative recombination.

Represents photo-excited carriers (electrons and holes) which recombineradiatively before being extracted and produce either a photon withenergy equal to the band-gap or possible multiple photons with energiesless than the band-gap. This radiated energy is not necessarily lost asthese photons can be reabsorbed. However, these radiated photons, unlessconfined, will be re-emitted from the cell back toward the incomingsunlight and lost forever—an effect that ultimately restricts themaximum efficiency that can be achieved by the solar cell. In most bulksemiconductor materials, the timescale of carrier recombination istypically less than a few microseconds (see FIG. 2). For the solar cellto be efficient, most photo-excited carriers must be transported to thecell contacts and extracted before the carriers recombine, although atthat point the energy separation of the electrons and holes to beextracted from the cell would only be comparable to the cell materialband-gap energy.

Represents photo-excited carriers (electrons and holes) which recombinenon-radiatively with the help of electronic states within the band-gap.These states are typically caused by defects in the solar cell materiallattice structure or by impurity atoms, and the resultant non-radiativecarrier recombination would produce phonons, thus the energy of theabsorbed solar photons that caused the excitation of these carriers istransferred to heat rather than being converted into current by thesolar cell. This loss mechanism is one of the main efficiency lossmechanisms in monolithic multi-junction stack solar cells where thelattice mismatch between successive layers can create lattice misfitdislocations which can severely diminish the solar cell performance bycreating additional regions at the stacked cells boundaries wherecarriers can non-radiatively recombine.

Represents photo-excited carriers (electrons and holes) which are noteffectively extracted by the solar cell contacts. This loss mechanism istypically caused by high resistance at the cell contacts that tends tocause inefficiency in extracting the carriers out of the cell, thusultimately limiting the maximum efficiency that can be achieved by thesolar cell. This mechanism is also an important efficiency lossmechanism in monolithic multi-junction stack solar cells as there areonly two contacts to extract the current from the multi-junction stack,making the lowest individual current producing cell structure within thestack limit the total current of the entire multi-junction stack. Alsothis loss mechanism is the main culprit behind the difficulty inextracting hot carriers from solar cells as these carriers tend torapidly cool down at the contact, an effect that causes the hot carriersto congregate near the cell junction, making it difficult to extractthese carriers before they cool down.

In addition to the above efficiency loss mechanisms, the theoreticalmodel typically used to predict solar cell efficiency, namely theSQ-Model, includes certain assumptions that limit the perceivedefficiency that can be achieved by solar cells—thus somewhat preventingsolar cell designers from pushing their designs to their true limits.The most relevant of these assumptions are listed below.

1. The input is the un-concentrated solar spectrum;

2. Each incident solar photon will produce only one electron-hole pair;

3. The cell can achieve only one Quasi-Fermi Level (QFL) separation;

4. The cell is operating at thermal equilibrium with the cell andcarrier temperatures being equal; and

5. The cell is operating in steady state current flow condition.

The solar cell efficiency limit based on the SQ-Model is calculated byexamining the amount of electrical energy that can be extracted perincident solar photon. Since the incident solar photon excites anelectron from the solar cell material valence band to its conductionband, only photons with more energy than the cell material band-gap willproduce power. That means that the theoretical conversion efficiency ofa silicon (Si) solar cell with band-gap at 1.1 eV would be less than 50%since almost half of the photons within the solar spectrum have energybelow 1.1 eV. Considering the difference in the energy between the solarphoton being absorbed from the sunlight at 6000° K and the celloperating at 300° K, the SQ-Model equilibrium assumption would implythat any solar photon energy above and beyond the cell material band-gapenergy would be lost. Since blue photons have roughly half of the solarenergy above 1.1 eV, the combination of these two assumptions wouldresult in a theoretical efficiency peak performance of approximately 30%for a single junction Si solar cell.

In addition to the efficiency limitations implied by the SQ-Modelassumptions, there are several other considerations that are implied bythe material system used in the solar cell, such as the carrierproduction rate and mobility characteristics of the material system.These types of considerations do not affect the efficiency of the cellunder normal conditions, but introduce further limits under certainconditions (for example, an increase in the number of incident solarphotons due to concentration). The first of these two effects, namely,the carrier production rate, sets a saturation (or a maximum) level onthe rate in which carriers are produced within the cell material as aresult of photo-excitation, and hence limits the amount of energy thatcan be extracted from the cell. Intuitively, as the number of solarphotons incident on the cell surface increases, the amount of energythat can be produced by the cell should increase. However, such is notthe case in some material systems (such as Si, for example) in which,due to low electron mobility, the number of holes increases with theincrease of photo-excitation at a rate that is much faster thanelectrons. This hole and electron density imbalance will causephoto-excited electrons to recombine with the abundantly available holesbefore they can be extracted, thus placing a limit on the number ofelectron/holes that can be extracted from the cell. In Si cells, thislimiting rate (equilibrium) is reached at less than 2-sun of incidentlight. As a result, when twice as much sunlight is incident on thesurface of a Si solar cell, the carrier production rate would only beslightly higher than with 1-sun, making the ratio of the input energy tooutput energy lower, which represents a much lower efficiency. For thatreason Si solar cells are not effective with solar concentrators.

The electron mobility in other material systems, such as galliumarsenide (GaAs) or gallium nitride (GaN), is much higher than that insilicon, enabling photo-excited electrons to reach the cell junctionmore quickly, thus alleviating the occurrence of holes/electron densityimbalance and reducing the chances that electrons and holes willrecombine before they can be extracted, which in turn would allow anincrease in the number of incident solar photons to continue to resultin an increase in the number of photo-excited carriers beforeequilibrium is reached. This increase in electron mobility, therefore,would allow solar cells made from such material systems to have anincreased efficiency under concentrated sunlight.

The discussion in the following sections of this disclosure is intendedto highlight several novel design approaches that would circumvent manyof the efficiency loss mechanisms explained above and therefore allowthe alternating bias solar cell designs described in the followingsections to offer extremely high solar power conversion efficiency.Subsequent sections of the disclosure will discuss the cost/efficiencyperformance of multiple embodiments of the alternating bias solar cellsand compare it with the performance achieved by current conventionalsolar cells. The objective of the discussion below is therefore to showthat the cost/efficiency performance predicted to be achieved by thealternating bias solar cell of this invention could offer a solar energycost per kWh that reaches the 3^(rd) Generation (3G) target of thephotovoltaic solar cell industry.

Harnessing Hot Carriers

As explained earlier, one of the primary loss mechanisms in solar cellsis the loss of incident solar photons with energy above the cellmaterial band-gap due to hot carrier relaxation, loss mechanism

in FIG. 1. Although it is theoretically possible for hot electrons to beseparated and collected at contacts before cooling occurs, this is notobserved in conventional solar cells due to the fast thermalization ofhot carriers (short cooling time, τ_(c)). Currently there are twoconcepts being envisioned by researchers in the field for increasingsolar cell efficiency utilizing hot carriers, namely, direct extractionusing selective energy contact (SEC) and multiple exciton generation(MEG). Both concepts rely on first slowing down the carrier cooling, butthe hot carrier energy is harnessed in different ways.

Theoretical treatments of direct hot carrier extraction using SEC, whichis illustrated in FIG. 3A (“Solar Energy Material and Solar Cells”, P.Würfel, 46 (1997), pp. 43-52), is widely published and have shown thatsubstantial solar cell efficiency increase near the thermodynamic limitof 68% from a single junction cell (“The Physics of Solar Cells” J.Nelson, Imperial College Press, 2003, pp. 309-316) would be possible ifhot carriers could be effectively extracted. However, it is not easy toseparate hot (high energy) electrons and holes (carriers) to the cellcontacts because these hot carriers tend to lose their high energythrough the interaction with phonons that cause the high energy of hotcarriers to be rapidly lost as heat. The entire concept behindmaintaining the photo-excited hot carrier population within the cell inboth SEC and MEG approaches is to minimize the electron-phononinteractions. However, in the vicinity of metal contacts, it is veryeasy for the hot carriers to cool down through the large number ofavailable electronic states in the contacts. Therefore, hot carrierswould typically tend to congregate near the cell junction, making iteven more difficult to transport and extract these carriers before theycool down. In typical solar cell materials, the distance the hotcarriers can travel through the cell material before cooling tends to bevery short (less than a micron), making it more difficult to transportthe hot carrier to the cell contacts before they cool down.

It should be noted that the principle of the SEC approach is to use acontact material having a narrow density of states with large band-gapbetween the next available states (“Solar Energy Material and SolarCells”, P. Würfel, 46 (1997), pp. 43-52). However, a narrow density ofstates would also yield extremely low electron mobility and thereforethere must be some level of compromise between the narrowness of thedensity of states and maintaining high enough conductivity through thecontact. An additional issue that will need to be addressed before SECbecomes feasible is the geometry of the cell and its associatedcontacts. Given that the distance the hot carriers can travel beforecooling is typically very short, it would be necessary to design thecell structure such that carriers are generated very close to the SECcontact to ensure the carriers do not cool before being collected at thecontact. Therefore, very short absorber regions and/or convolutedsurfaces may be required to minimize the distance the hot carriers willhave to travel (“Third Generation Photovoltaics: Advanced Solar EnergyConversion”, M. A. Green, Springer, New York, 2003, pp. 35-43).

The other possibility for increasing the efficiency of solar cellsutilizing hot carriers is through MEG (“Third Generation PhotovoltaicsAdvanced Solar Energy Conversion”, M. A. Martin, Springer 2006, pp81-88). In this case, the excess energy of the hot electrons is used tocreate additional excitons, i.e., bound electron-hole pairs. The hotelectron must have the energy of at least two times the band-gap E_(g)to create one additional electron-hole pair. This process is not limitedto electrons with energy of twice the band-gap, but it can also beextended to electrons with higher energies. Under 1-sun AM1.5 spectrum,the predicted theoretical efficiency of MEG-enhanced cells is over 44%,while under maximum sunlight concentration, the efficiency can approachthat of SEC cells. Although MEG can occur in bulk semiconductors, itsprobability of occurrence is so low that it does not contribute much tothe efficiency of the cell (“Third Generation Photovoltaics”, Gregory F.Brown and Junqiao Wu, Laser & Photon Rev., 1-12 (2009), publishedonline: 2 Feb. 2009).

As stated earlier, slowing the cooling of hot carriers is prerequisitefor both the SEC and MEG approaches and the most widely pursued way forachieving this by ongoing research in the field is through the use ofquantum confinement structures. There are cases wherein hot carriers'cooling time exceed the typical cooling time in bulk semiconductors.This phenomenon is expected to occur in many material systemsincorporating quantum confinement structures. First, multiple quantumwells (MQWs) and quantum dots (QDs) were studied and found to have hotcarrier cooling times much larger than that of bulk semiconductors(“Third Generation Photovoltaics”, Gregory F. Brown and Junqiao Wu,Laser & Photon Rev., 1-12 (2009), published online: 2 Feb. 2009). Hotcarrier cooling times approaching a few tens of nanoseconds have beenobserved in these types of structures. This increase has been attributedto the phenomenon known as the phonon bottleneck effect in quantumstructures. Typically hot electrons cool through interactions withoptical phonons and due to the presence of quantum confinement, anon-equilibrium level of optical phonons can be created. Due to thephonon bottleneck effect caused by the quantum confinement aspects ofMQWs or QDs, these optical phonons cannot equilibrate with the latticefast enough, thereby slowing the further cooling of hotelectrons—extending their cooling time τ_(c) (see FIG. 2). Within thetwo dimensional quantum confinement of MQWs, the phonon bottleneckeffect occurs at high carrier photo-excitation densities that requirerelatively high illumination levels, such as those typically occurringunder sunlight concentration. However, due to the three dimensionalquantum confinement aspects of QDs, the phonon bottleneck effect isexpected to occur under all illumination levels. As a result of theslowed down cooling of hot carriers in MQWs and QDs, these types ofdevice structures are expected to play an important role in hot carrierextraction.

In principle, in the SEC hot carrier cell illustrated in FIG. 3A, withthe hot carrier cooling being slowed down by the quantum confinementstructures incorporated with the cell material, there would besufficient time to transport the carriers to the contacts while they arestill hot where they can be collected at their high energy level by thenarrow density of states of the contacts. This would theoretically allowan increase in the photovoltage yielded by the cell. However, theincreased photovoltage at the cell contacts would tend to counteract thebuilt-in potential V_(bi) responsible for transporting the carriers tothe cell contacts. As a result, the time it would take to transport thecarriers to the contacts (carrier extraction time) would besubstantially increased to the point that could reach the carrierrecombination time τ_(r); meaning the carriers would recombine beforereaching the contacts due to the weakening of the cell carrier built-intransport mechanism, which in turn would result in a substantialreduction in the photocurrent yielded by the cell with net energyextraction that is not substantially greater than that of a conventionalcell. So although SEC hot carrier cells can in theory generate higherphotovoltage, the realized increase would most likely be more thanoffset by the reduction in photocurrent that is a direct consequence ofthe prolonged carrier extraction time.

Given the increased attention to renewable energy, in particularphotovoltaic (PV) solar cells, there is an increasing demand to increasethe efficiency of PV cells without substantially increasing their cost.Hot carrier PV solar cells have been theoretically predicted to be ableto offer a substantial increase in the PV cell efficiency, but to datenone of these predictions have been realized. The two hot carrier solarcell approaches discussed earlier require means for increasing thecarrier cooling time, which require the inclusion of quantum confinementstructure within the cell material, which in turn would very likelyincrease the cell cost. The benefits of MEG hot carrier cells can onlybe realized under very high solar concentration that renders thatapproach impractical. In addition to requiring special types of contactsthat often require the use of a multi layer superlattice, the SEC hotcarrier solar cell approach appears to suffer from a built-in deficiencythat counteracts its ability to reach a higher energy efficiency thanwhat a conventional PV cell can offer. Given the high demand for moreefficient and less costly PV solar cells and the weaknesses of theapproaches currently being pursued to attain this objective, a PV solarcell approach that can effectively realize higher efficiency withoutsignificant increase in the solar cell will most likely have asubstantial market value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the photovoltaic solar cell efficiency lossmechanisms.

FIG. 2 illustrates the photovoltaic solar cell efficiency lossmechanisms.

FIG. 3A illustrates the energy band structure of prior art photovoltaicsolar cells that use selective energy contacts.

FIG. 3B illustrates the electron flow in conventional fixed bias regimesolar cells.

FIG. 4 illustrates the current-voltage (I,V) characteristics of aconventional p-n junction solar cell in the dark and under illumination.

FIG. 5A presents a high level block diagram of the preferred embodimentof the variable bias hot carrier solar cell of this invention.

FIG. 5B illustrates the waveform of the photovoltage of the variablebias hot carrier solar cell of this invention.

FIG. 5C presents a detailed block diagram of the hot carrier solar cellof this invention.

FIG. 6 illustrates the expected (I,V) characteristics of the hot carriersolar cell of this invention.

FIG. 7A presents an exemplary block diagram of the hot carrier solarcell of this invention having a direct current (DC) output.

FIG. 7B illustrates an exemplary block diagram of the hot carrier solarcell of this invention having an alternating current (AC) output.

FIG. 7C illustrates an exemplary block diagram of the self biased hotcarrier solar cell of this invention.

FIG. 7D represents an exemplary block diagram of an alternate hotcarrier solar cell of this invention having a direct current (DC)output.

FIG. 8 illustrates an exemplary implementation of the alternating biashot carrier solar cell of this invention.

FIG. 9A presents a high level block diagram of an alternative embodimentthe hot carrier solar cell of this invention.

FIG. 9B illustrates the waveform of the photovoltage of the alternatingserial bias of the alternative embodiment of the hot carrier solar cellof this invention.

FIG. 9C illustrates the waveform of the photovoltage of the pulsedparallel bias of the alternative embodiment of the hot carrier solarcell of this invention.

FIG. 9D presents a typical block diagram of the parallel bias circuit ofthe alternative embodiment of the hot carrier solar cell of thisinvention.

FIG. 10A illustrates the energy band structure of the core solar cell ofthe hot carrier solar cell of this invention that incorporates quantumconfinement means.

FIG. 10B illustrates the intermediate bands of the core solar cell ofthe hot carrier solar cell of this invention that incorporates quantumconfinement means.

FIG. 11A illustrates a cross sectional view of the core solar cell ofthe hot carrier solar cell of this invention that incorporates opticalconfinement means.

FIG. 11B illustrates a cross sectional view of the core solar cell ofthe hot carrier solar cell of this invention that incorporates bothoptical and quantum confinement means.

FIG. 12 illustrates the candidate material systems for the alternatinghot carrier solar cells of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

References in the following detailed description of the presentinvention to “one embodiment” or “an embodiment” mean that a particularfeature, structure, or characteristics described in connection with theembodiment is included in at least one embodiment of the invention. Theappearance of the phrase “in one embodiment” in various places in thisdetailed description is not necessarily all referring to the sameembodiment.

Rather than relying on slowing down the hot carrier cooling as in thedescribed prior art, the approach for harvesting hot carriers to bedescribed herein relies instead on accelerating the hot carrierextraction. The approach for hot carrier extraction described hereinstems from the observation that if the carrier extraction is fastenough, it may be possible to extract the carriers before their highenergy is lost to heat. This could be accomplished if the intensity ofthe electric field responsible for transporting the carriers to thecontacts can be instantaneously increased within the duration of the hotcarrier cooling time τ_(c), meaning before the carriers cool down. Theeffect of such an increase in the intensity of electric field would beto increase the transport velocity of the hot carriers to the extentthat the carrier extraction time would become shorter than the carriercooling time τ_(c), thus enabling the extraction of carriers while theyare still “hot”; meaning while the carriers still have an energy levelabove the edge of the cell band-gap If this can be accomplished, itwould most certainly be much easier to implement at the device levelthan the currently predominant approach being pursued of using complexsuperlattice contacts in the vicinity of where the hot carriers areconcentrated, which has proven to be geometrically fairly difficult toachieve. The hot carrier extraction capabilities of such an approachwould not be restricted by any of the selective energy aspects of acomplex superlattice contact explained earlier, and would not impose anygeometric constraints that would complicate the solar cell devicestructure. Rather, it would be applied in conjunction with conventionalbulk material as well as quantum confinement based solar cells withadditions only at the circuit level rather than at the device materiallevel. The following discussion provides further details on theapplication of this approach in bulk material cells, for the remainderof this section, and in cells incorporating quantum confinementstructures, for which the carrier acceleration approach described hereinis equally applicable, in the subsequent sections.

Without illumination, bringing the n-doped and the p-doped sides of asolar cell into contact causes a temporary current flow of electronsfrom the n-doped side to the p-doped side of the cell to offset thecontact potential caused by the difference in the Fermi levels of thetwo sides of the cell junction; conventionally known as the built-inpotential V_(bi) of the junction. This temporary current flow stops oncethe electric field formed by the diffused charges at the cell junctionoffsets the contact diffusion force acting on the electrons and holes.Under illumination, the photo-excitation caused by the incident solarphotons causes an increase in the carrier population density within thecell as photo-excited electrons from the valence band of the p-dopedside of the cell are promoted to the conduction band. With this increasein photo-excited carrier population within the cell, the built-inelectric field ε_(bi) of the cell separates the photo-excited carriersand causes electrons and holes to move toward the cell n-contact and pcontact, respectively. When a load is connected across the two sides ofthe cell, as illustrated in FIG. 3B, the photo-excited electrons flowwithin the cell from the direction of the p-doped side to the n-dopedside of the cell where they are extracted at the n-doped side contactand flow into the connected load, lose their energy while moving intothe load, and return back to the cell through the contact at the p-dopedside of the cell where they recombine with the awaiting holes in thep-doped side of the cell. This forward bias flow of electrons within thesolar cell and the connected load is illustrated in FIG. 3B.

Referring to FIG. 3B, depending on the value of the load resistanceR_(Load) across the cell contacts, the photovoltage built-up at the cellcontacts will cause an electric field ε_(pv) across the cell junctionthat counteracts the cell internal electric field ε_(bi) responsible forthe carrier transport effect. As the photovoltage across the cellcontacts increases, the cell internal electric field ε_(bi) caused bythe cell built-in potential V_(bi) becomes weakened by the opposingelectric field ε_(pv) causing photovoltage build-up at the cellcontacts. As a consequence the photo-excited carrier transport towardsthe cell contacts is systematically weakened making a lesser number ofphoto-excited carriers able to reach the cell contacts before theyrecombine. This effect is detrimental to hot carrier solar cells sincethe cell transport mechanism is primarily the cell internal electricfield ε_(bi) caused by the cell built-in potential V_(bi) while theenergy of the electrons to be extracted from the cell, and therefore thephotovoltage across the cell contacts they are expected to generate, isnow much higher than the photovoltage of a conventional solar cell. Asexplained earlier, the higher photovoltage sought after from hot carrierextraction weakens the built-in carrier transport mechanism of the cellleading to a reduced photocurrent, thus rendering the higher powerextraction efficiency of hot carrier solar cells unattainable.

FIG. 4 illustrates the current-voltage (I,V) characteristics of aconventional p-n junction solar cell in the dark (405) and underillumination (410). In order to maximize the yielded output power fromtoday's conventional single junction solar cells, the value of the loadresistance R_(Load) is typically selected at a balance point between thecounteracting electric fields ε_(bi) and ε_(pv), namely, the electricfields caused by the cell built-in potential V_(bi) and the photovoltagebuilt-up at the cell contacts, that will achieve the maximum yieldedphotovoltage (V_(m)) and photocurrent (I_(m)) from the cell. The maximumyielded photovoltage (V_(m)) and photocurrent (I_(m)) are typicallyachieved by the cell at a bias in the vicinity of knee point (415) ofthe (I,V) curve of FIG. 4. In order to achieve these maximumphotovoltage V_(m) and photocurrent I_(m) values, the balance betweenthe two counteracting electric fields ε_(bi) and ε_(pv) must leave acarrier transport force that is sufficient to transport the maximalnumber of photo-excited carriers to the contacts before the carriersrecombine. With a typical value of cell built-in potential V_(bi) of ˜1volt generating an internal electric field across the junction depletionregion of a few microns, the carrier transport velocity (alsoconventionally known as the drift velocity) can reach saturationvelocity typically in the range ˜10⁷ cm/s across the cell depletionregion. When the photovoltage across the cell contacts is at a minimalvalue, this level of carrier transport velocity can readily transportthe photo-excited carriers to the cell contacts well before the carriersrecombine. This is clear in FIG. 4 which shows the photocurrentgenerated by the cell is at its maximum value when the cell photovoltageis below the knee point (415) of the (I,V) curve of FIG. 4. However,when the photovoltage across the cell contacts is at the highestpossible value, which is typically in the range of the cell band-gap(typically 1.1 eV in silicon and 1.4 eV in gallium arsenide), theresultant electric field across the cell contacts ε_(pv) weakens thecell internal electric field ε_(bi) to the extent that the carrierstransport to the cell contacts all but stops and the cell photocurrentdiminishes to a minimal value as the carriers recombine before reachingthe cell contacts and getting extracted from the cell. This is clear inFIG. 4 which shows the photocurrent generated by the cell quicklydiminishes to a minimal value when the cell photovoltage is above theknee point (415) of the (I,V) curve of FIG. 4. This condition isseverely exacerbated in hot carrier solar cells since the main objectivebehind such cells is to extract hot electrons with energy substantiallyhigher than the cell material band-gap energy because when the cellphotovoltage reach values substantially higher than the cell materialband-gap energy, the cell internal field ε_(bi) responsible for carriertransport would have been sufficiently weakened to the point that nophoto-excited carrier at any energy level can reach the cell contacts.

A high level block diagram of the preferred embodiment of the hotcarrier solar cell design of this invention is illustrated in FIG. 5A.This preferred embodiment of the hot carrier solar cell of thisinvention overcomes the drawback encountered in prior art hot carriercell designs described earlier by allowing the photovoltage generated bythe cell to vary intermittently between a minimum (V_(min)) and amaximum value (V_(max)) such that the average transport velocity ofphoto-excited carriers across the cell is maintained at a value thatwould result in the transport of the photo-excited carriers to the cellcontacts before the carriers cool down, meaning within a time durationthat is shorter than the carrier cooling time τ_(c). The minimum V_(min)value of the alternating photovoltage of the hot carrier solar cell 500design of this invention would be selected at a point the cell internalbuilt-in electric field ε_(bi) is still at its highest value, meaningthe point at which the photo-excited carrier transport velocity canreach its maximum value. The maximum value V_(max) of the alternatingphotovoltage of the hot carrier solar cell 500 of this invention wouldbe selected at a value that is substantially commensurate to the maximumvalue of the electrochemical potential of the photo-excited carrierswithin the solar cell 500. (The electrochemical potential is the energyseparation between the Quasi Fermi levels of the semiconductor materialcaused by the photo-excitation by solar photons.) Such maximum valueV_(max) of the alternating photovoltage of the hot carrier solar cell500 of this invention would be selected at the highest photovoltage thatcan be achieved by the cell without regard to the counteracting effectsuch a high photovoltage value would have on diminishing the value ofthe cell internal built-in electric field ε_(N). The possible range ofthe values of V_(min) and V_(max) relative to the (I,V) characteristicsof a conventional solar cell is illustrated in FIG. 4 with referencenumber 420. The alternation of the photovoltage of the hot carrier solarcell 500 of this invention between the minimum V_(min) and maximumV_(max) values selected based on the above criteria would result inalternating time periods during which the photo-excited carriertransport velocity across the cell reaches maximum and minimum values,respectively. Depending on the duty cycle of the alternation between theminimum and maximum values of the photovoltages of the hot carrier cellof this invention, the resultant average transport velocity of thephoto-excited carriers can be sustained at a value that would provide acontinuous transport of carriers toward the cell contacts, and hencephotocurrent, even when the cell photovoltage is at the maximum value ofits alternation cycle.

FIG. 5B illustrates the waveform representing the alternation of thephotovoltage of the hot carrier cell 500 of this invention. Asillustrated in FIG. 5B, the alternation of the photovoltage of the hotcarrier solar cell 500 of this invention between its minimum V_(min) andmaximum V_(max) values would have a time duration T_(b). The first keyparameter of the waveform illustrated in FIG. 5B that represents thealternation of the photovoltage of the hot carrier cell 500 of thisinvention is the percent of the cycle T_(b) during which thephotovoltage is allowed to reach its minimum V_(min) value, which isrepresented in FIG. 5B as (αT_(b)) would have to be maintained longenough to sustain the needed average transport velocity for thephoto-excited carriers yet short enough to keep the average photovoltageachieved by the hot carrier cell 500 at the highest possible value. Thedominant portion of the photo-excited carrier transport to the contactsof the hot carrier solar cell 500 of this invention will take placeduring the time period (αT_(b)) of the photovoltage alternation cycleT_(b) when the internal built-in field ε_(bi) of the cell is allowed toincrease to its maximum value during the alternation cycle T_(b). Sinceas stated earlier, when the cell photovoltage is at its minimum valueV_(min), the photo-excited carriers transport velocity can reach ˜10⁷cm/s during the time period (αT_(b)), which is sufficient to transportphoto-excited carriers close to 100 μm in 1 ns. In taking into accountthe transient effect of the carrier transport velocity build up anddecay during the time period (αT_(b)), it is reasonable to assume thatthe average transport velocity of the photo-excited carriers during thetime period could only reach ˜0.1×10⁷ cm/s, which is still sufficient totransport photo-excited carriers close to 10 μm in 1 ns. Which meansthat if the sub-interval of the photovoltage alternation period duringwhich the cell photovoltage reaches its minimum value V_(min) (αT_(b))=1ns, the cell internal field ε_(bi) will be able to transport thephoto-excited carriers within that time interval close to 10 μm. Thisimplies that the time period (αT_(b)) can be selected depending on theaverage distance the photo-excited carriers have to be transported tothe cell contacts. For example, in gallium arsenide (GaAs) singlejunction solar cells, the typical thickness between the cell emitter andbase layers can be less than 5 μm, which implies that the photo-excitedcarriers in a GaAs solar cell will have to travel an average distance of2.5 μm to be extracted at the cell contacts. This means that for a GaAssolar cell, (αT_(b))=0.25 ns will be sufficient to allow enough time totransport most all of the photo-excited carriers within the cell 500 tothe contacts within the hot carrier cooling time τ_(c). Comparablevalues (αT_(b)) would apply for thin-film type solar cells such ascadmium telluride (CdTe), copper indium diselenide (CIS) and copperindium gallium diselenide (CIGS), since the typical thickness betweenthe cell emitter and base layers of these types of solar cells can alsobe less than 5 μm. For indirect band-gap solar cells, such as silicon(Si) and germanium (Ge) solar cells, due to the larger light absorptionlength of these cells, the typical thickness of the cell emitter andbase layers can be much larger than GaAs, CdTe and CIGS solar cells.However, the indirect band-gap, carrier mobility and crystal latticecharacteristics of these cells can make the hot carrier cooling timeτ_(c) in these cells be at least one order of magnitude longer than thatof GaAs, CdTe and CIGS solar cells. In Si single junction solar cells,the typical thickness of the cell emitter and base layers can be 300 μm,which implies that the photo-excited carrier in a Si solar cell willhave to travel an average distance of 150 μm to be extracted at the cellcontacts. This means that for a Si solar cell, (αT_(b))=15 ns will besufficient to allow enough time to transport most all of thephoto-excited carriers within the cell 500 to the contacts within thehot carrier cooling time τ_(c).

It should be noted that although the value of carrier transport time ishigher for Si solar cells, it is expected the hot carrier cooling timeτ_(c) in Si will also be longer as well. Nonetheless, the subsequentdiscussion in this disclosure will show that the carrier transport timecan be made substantially shorter when light confinement means areincorporated within the Si solar cell structure which would allow thecontact-to-contact thickness in the Si cell to be substantially reduced.For such thin-Si solar cells that incorporate light confinement means, a20 μm thick silicon film would have much higher absorptance than a 400μm thick Si cell without light confinement means (“Physics of SolarCells”, Würfel, pp. 173-177). Furthermore, in thin-Si solar cells thatincorporate light confinement means which also incorporate buriedcontacts, to be described in subsequent discussion, the distance betweenthe cell contacts can be made to be on the order of 5 μm, which would inturn make the carrier transport time for this type of cell comparable tothat of GaAs, CdTe and CIGS solar cells. This means that for a Si solarcell incorporating light confinement means (αT_(b))=0.25 ns can also bemade possible.

The second key parameter of the waveform illustrated in FIG. 5B thatrepresents the alternation of the photovoltage of the hot carrier cell500 of this invention is the cycle T_(b) during which the photovoltagegoes through a full cycle from its minimum V_(min) to maximum V_(max)values. With the ability to transport the photo-excited carriers withinthe cell during the minimum photovoltage period (αT_(b)), what remainsis to select the cycle T_(b) to be substantially equal to or shorterthan the carrier cooling time τ_(c). Since as explained earlier the hotcarriers cooling happens, depending on the cell material crystal latticecharacteristics, in a timescale of a few nanoseconds, the alternation ofthe photovoltage of the hot carrier cell of this invention, namely, thecycle T_(b), can also be selected to be on the order of a fewnanoseconds as well. By selecting the period (αT_(b))=0.25 ns for GaAs,CdTe, CIGS and thin-film Si types of solar cells, a value of theparameter α=0.1, will result in a value for T_(b)=2.5 ns, which is shortenough to ensure that the hot carrier generated by the cell within thecycle time T_(b) would not have the chance to cool down before theoccurrence of the sub-cycle (αT_(b)) during which substantially all ofthe hot carriers will be transported to the contacts of cell 500.

It should be noted that photo-excited carriers transport will continueto occur at varying transport velocities throughout the entire cycleT_(b) with carriers that reach the cell 500 contacts at differentinstants within the cycle T_(b) getting extracted at an energy levelproportional to their energy level since the photovoltage across thecell 500 contacts is made to change during the cycle T_(b) over a rangeof values that extend from below the band-gap energy of the cell to thedesired maximum value corresponding to the energy of the hot carriers tobe extracted from the cell 500. For conventional Si solar cells (meaningthose not incorporating light confinement means or buried contacts), alarger value of the parameter a can be selected, for example α=0.5, thatwould result in a value for T_(b)=30 ns, which could be sufficient toensure that a substantial number of the photo generated hot carriers canbe extracted from the cell before cooling since, as explained earlier,the hot carrier cooling time τ_(c) in Si based cells is expected to besubstantially longer (close to one order of magnitude) than that inGaAs, CdTe, and CIGS based solar cells.

The variability of the photovoltage of the hot carrier cell 500 of thisinvention during the cycle T_(b) from its minimum V_(min) to maximumV_(max) values allows the extraction of photo-excited carriers across anextraction energy range that can be made to substantially match theenergy profile of the photo-excited carriers generated within the cellthat spans from the band-gap energy of the cell up to the maximum energylevel as defined by the selected maximum value the photovoltage of thecell 500 is allowed to reach during the cycle T_(b). This is adistinctive feature that is unique to the hot carrier cell 500 of thisinvention, since all current conventional single junction photovoltaicsolar cells can extract photo-excited carriers from the cell only at asingle energy level. Only multi-junction solar cells can extractphoto-excited carriers over a wide range of energy levels using costlystacks of p-n junctions, and even then at a single energy level perjunction layer. In contrast, the hot carrier cell 500 of this inventioncan extract photo-excited carriers across a wide range of energy levelsand using only a single junction. Due to the alternation of itsphotovoltage, the hot carrier solar cell 500 of this invention can bethought of as a solar cell that temporally sweeps through a wide rangeof extraction energies at a rate that is comparable to or faster thanthe carrier cooling rate τ_(c), thus allowing carriers to be extractedfrom the cell not only before cooling but also at an energy level thatis commensurate with their energy level. It is also worth mentioningthat since, as explained earlier, the carrier extraction energy in thehot carrier cell 500 of this invention cycles through a wide range ofenergy levels within the hot carrier cooling time τ_(c), the hotelectron/hole pair (carrier pair) that reaches the cell contacts at agiven value of electrochemical potential (energy separation) can also betransferred to the cell 500 load before they cool down at the contactsince the instantaneous extraction energy difference between the cell500 contacts will match the hot electron/hole pair energy levelseparation within the carrier pair cooling time interval τ_(c). Thismeans that at any given instant of the cycle T_(b), the instantaneousphotovoltage of the cell 500, and hence potential separation between itscontacts, would match the energy level separation of some of the hotelectron/hole pairs photo-excited within the interval time intervalT_(b)≦τ_(c), thus allowing such carrier pairs to be transferred from thecell to the load through a contact having a matched energy separationbefore the decay of their energy level separation. This feature makesthe hot carrier cell 500 of this invention to not require complexselective energy contacts to extract the hot carriers out of the cell.This is made possible because the alternating photovoltage of the hotcarrier cell 500 of this invention makes available at any discreteinstant of time within the alternation cycle T_(b) of the cellphotovoltage (which is comparable in duration to or shorter than the hotcarrier cooling time interval τ_(c)) an instantaneous and temporallydiscrete narrow extraction energy band at the cell contacts that lastsfor a time interval that is substantially shorter than the hot carriercooling time interval τ_(c) which is also made available cyclically at arate T_(b) that is equal to or shorter than the carrier's cooling timeτ_(c). In other words the extraction energy levels at the contacts ofthe hot carrier cell 500 of this invention are made to be temporallyenergy selective as the photovoltage of the cell is alternated at a ratethat is faster than the hot carrier cooling rate. In addition, beyondbeing temporally energy selective, the extraction energy levelseparation between the contacts of the hot carrier cell 500 is also madeto temporally vary to cover a wide energy band that would span theextent from the cell band-gap energy to a desired energy level that issubstantially higher than the cell band-gap energy. These uniquefeatures of the hot carrier solar cell 500 of this invention in effectwould allow the cell energy extraction efficiency benefits of amulti-junction solar cell from a single junction solar cell at asubstantially lower cost.

As illustrated in FIG. 5A, the photovoltage of the hot carrier cell 500of this invention is made to vary in accordance with the waveformillustrated in FIG. 5B by incorporating either the bias circuits 510 or520 in series or in parallel, respectively, with the core solar cellelement 530. Either of the bias circuits 510 or 520 can be implementedeither as an integrated circuit device or a discrete component circuitboard that can be integrated with conventional GaAs, CdTe, CIGS or Sibased solar cells. In order to cause the photovoltage of the cell totemporally vary as illustrated in FIG. 5B, the bias circuits 510 or 520would have to cause the effective resistance across the contacts of thecore solar cell 530 to also temporally vary in such a way that wouldcause the photovoltage across the contacts of the core solar cell 530 tofollow the waveform illustrated in FIG. 5B. Without loss of generality,the remaining discussion will focus on the detailed description of theseries bias circuit 510 since the design of the parallel bias circuit520 would be substantially similar, albeit with a different set ofdesign parameters. A person skilled in the art can readily utilize thedetailed description of the series bias circuit 510 provided herein toselect the design parameters of the parallel bias circuit 520.

FIG. 5C illustrates an exemplary detailed block diagram of the hotcarrier solar cell 500 of this invention that utilizes the series biascircuit incorporated within the dashed line block 510 in FIG. 5C. FIG.5C shows the bias circuit 510 connected in series with the core solarcell 530, which can be either a GaAs, CdTe, CIGS or Si based p-njunction solar cell. The bias circuit 510 illustrated in FIG. 5C isbasically a temporally varying resistor R_(v) that is comprised of anoscillator 550, a diode 560 and multiplicity of resistors and capacitorsmarked accordingly on FIG. 5C. The function of the oscillator 550 is togenerate a variable voltage signal v_(in) having a frequency f_(s) whosevalue equals the reciprocal value of the desired alternation cycle T_(b)of the photovoltage V_(out) of the hot carrier solar cell 500 of thisinvention or f_(s)=(T_(b))⁻¹. For the design examples discussed earlier,when the value of T_(b)=2.5 ns is selected for implementation in a GaAs,CdTe, CIGS or thin-film Si based hot carrier solar cell of thisinvention, the frequency f_(s) that needs to be generated by theoscillator 550 would be f_(s)=400 MHz. For the design example discussedearlier, when the value of T_(b)=15 ns is selected for implementation ofthe hot carrier cell 500 of this invention in conjunction with aconventional Si based cell, the frequency f_(s) that needs to begenerated by the oscillator 550 would be f_(s)=66.7 MHz.

The values of the resistor and capacitor pair (R₁,C₁) together with the(I,V) characteristics of the diode 560 would be selected to realize themaximum and minimum values of the variable resistance R_(v) needed tocreate the required maximum and minimum values, respectively, of thephotovoltage V_(out) across the contacts of the core solar cell 530. Thevalues of the resistor and capacitor pair (R₂,C₂) together with the(I,V) characteristics of the diode 560 would be selected to realize theratio α which sets the duty cycle of the sub-interval (αT_(b)) relativeto the cycle duration T_(b). During one cycle of the voltage v_(in)generated by the oscillator 550, the time variation of the voltagev_(in) will cause the effective resistance across the diode 560 tocyclically change, which will in turn cause the effective resistanceR_(v) of the entire bias circuit 510 to change cyclically as well from aminimum value R_(vmin) to a maximum value R_(vmax). This cyclical changeof the effective resistance of the bias circuit 510, when taken intoaccount together with the value of the load resistance R_(L), will causethe photovoltage V_(out) of the exemplary implementation of FIG. 5C ofthe hot carrier solar cell 500 of this invention to also cyclicallychange following the waveform illustrated in FIG. 5B. A person skilledin the art would know that the desired effect of implementing atemporally varying resistor can be realized in many alternative waysother than that described earlier, but the end effect would be the same.

The type of serial bias circuit 510 illustrated in FIG. 5C is similar tothose typically used in wireless applications which can be designed togenerate modulated signals well within the range of frequencies neededfor the implementation of the alternating bias solar cell 500. A personskilled in the art would know that there are many alternative circuitdesigns other than that illustrated in FIG. 5C that can be used togenerate the bias waveform illustrated in FIG. 5B with a comparable endresult.

FIG. 6 is an illustration of the photovoltage and photocurrent (I,V)characteristics expected to be yielded by the hot carrier solar cell 500of this invention. It should be noted that the photovoltage andphotocurrent achieved by the hot carrier solar cell 500 are in actualitythe photovoltage and photocurrent provided by the core solar cell 530when dynamically biased, as explained in the earlier discussion.Referring to FIG. 5C, the photovoltage and photocurrent achieved by thehot carrier solar cell 500 are therefore the voltage across and thecurrent through, respectively, the load resistor R_(L). As explainedearlier, when the photovoltage V_(out) of the hot carrier solar cell 500of this invention is made to temporally vary following the waveformillustrated in FIG. 5B, the effective extraction energy at the contactsof the core solar cell 530, which will be referred to as E_(out), willalso temporally vary following the waveform illustrated in FIG. 5B. FIG.6 shows a group of curves 611, 612, 613, 614, 615 and 616, eachrepresenting the expected (I,V) characteristics that would be yielded bythe core solar cell 530 when the extraction energy at its contacts is atthe values of E_(out) that ranges from 1.5 eV to 2.5 eV. The expected(I,V) characteristics to be yielded by the hot carrier solar cell 500 ofthis invention are illustrated in FIG. 6 as the envelope 620 of thegroup of curves 611, 612, 613, 614, 615 and 616, which represent thephotovoltage and photocurrent expected to be achieved by the hot carriersolar cell 500 when the voltage across its core solar cell 530 is sweptacross a set of values within the range of photovoltage from V_(min) toV_(max).

As illustrated in FIG. 6, the (I,V) characteristics expected to beyielded by the hot carrier solar cell 500 of this invention would extendover a wide range that is enabled by the wide range of extraction energyE_(out) values at the cell 500 contacts that extend from slightly belowthe band-gap of the core solar cell 530 to the value of E_(out) thatcorresponds to the maximum photovoltage V_(out) of the hot carrier solarcell 500 that is enabled by the incorporated bias cell 510. For example,since the rate at which the extraction energy E_(out) at the contacts ofthe hot carrier solar cell 500 is made to vary at a rate that iscomparable to or faster than the hot carriers cooling rate, at theinstant within the photovoltage variation cycle T_(b) of the hot carriersolar cell 500 of this invention when the instantaneous value of theextraction energy E_(out) at the contacts of the cell 500 is at 1.7 eV,the photovoltage V_(out) of the cell 500 would be at ˜1.15 V with aphotocurrent value that represents the number of carriers extracted bythe cell 500 which were photo-excited by solar photons at an energyvalue of E_(out)=1.7 eV. Similarly, at the instant when the cell 500photovoltage V_(out)˜1.35 V, the value photocurrent of the hot carriercell 500 would represent the number of carriers extracted by the cell500 which were photo-excited by solar photons with an energy value atthe value of E_(out)=1.9 eV, and so on. In effect, the variable bias ofthe hot carrier solar cell 500 of this invention would enable the cellto extract carriers photo-excited by solar photons with energy thatextends over a wide range and before the energy of these photo-excitedcarrier is lost due to the carrier cooling effect, thus allowing the hotcarrier cell 500 of this invention to yield values of photovoltage andphotocurrent, represented by the envelope 620, that are substantiallyhigher than what would be offered by the core cell 530 standaloneoperating at a static (fixed) conventional bias value.

As explained earlier, the photovoltage and photocurrent of the hotcarrier solar cell 500 of this invention will temporally vary at aprofile that is substantially comparable to the waveform illustrated inFIG. 5B. In order to make use of the variable output of the hot carriersolar cell 500, its output will have to be converted either to DC or ACformat. The conversion of the hot carrier solar cell 500 of thisinvention to DC format can be accomplished by mixing the photovoltageV_(out) of the hot carrier solar cell 500 with the output v_(out) of thebias circuit 510 in order to down convert it to baseband in very muchthe same way as the down conversion of the signal received by a radio isconverted from radio frequency (RF) band to baseband. This can beaccomplished, as illustrated in FIG. 7A, by adding the mixer 540 at theoutput of the hot carrier solar cell 500. The overall configuration ofthe hot carrier cell of this invention, which is now designated as 700in FIG. 7A, will then be comprised of the core solar cell 530 with thebias circuit 510 connected in series with it and the mixer 540 connectedat their collective output. It should be noted that the overallconfiguration of the hot carrier cell 700 of this invention canequivalently be implemented using the bias circuit 520 connected inparallel with the core solar cell 530 as illustrated in FIG. 5A.

Alternatively the output of the hot carrier solar cell 700 of thisinvention can be converted to AC format, as illustrated in FIG. 7B, byfirst mixing the output v_(out) of the bias circuit 510 with anoscillator signal having the same frequency as the desired AC formatthen further mixing the mixed version of the output v_(out) with thephotovoltage V_(out) of the hot carrier solar cell 500 in order to downconvert it to the desired frequency of the AC format. For example, if itis desired to make the output of the hot carrier solar cell 700 of thisinvention be in 60-Hz AC format, the output v_(out) of the bias circuit510 will first be mixed with the output signal of a 60-Hz oscillator 745using the mixer 750 then the resultant signal is further mixed using themixer 540 with output V_(out) of the hot carrier solar cell 500 toproduce 60-Hz AC format output from the overall hot carrier solar cell700. This unique feature of the hot carrier solar cell 700 which allowsits output to be converted to either AC or DC format is enabled by thealternating bias aspects of the hot carrier solar cell 500. It is worthnoting that the difference between the mixing circuits included in theconfigurations of FIG. 7A and FIG. 7B of the hot carrier solar cell 700to make its output either DC or AC, respectively, is not significant incomplexity. Which in turn implies that the DC and AC configurations ofFIG. 7A and FIG. 7B of the hot carrier solar cell 700 can be made tohave substantially the same cost and achieve substantially the samesolar power conversion efficiency. This is a substantial differentiationof the hot carrier solar cell 700 of this invention when compared totoday's conventional solar cells which typically require inverters toconvert its DC output to AC with an added cost plus a 25% reduction inthe overall cell efficiency.

Another biasing scheme that might be used is shown in FIG. 7D, whichillustrates both the desired object and a desired result. As showntherein the core solar cell 530 is connected between ground and anoutput circuit 710 which in turn is connected to a Load. The outputcircuit 710 comprises a high frequency form of Switching Regulator 740having, among other things, a Voltage Control 730 to control the(V_(min), V_(max)) voltage values that control the switching of theSwitching Regulator 740 so that it presents a variable non-dissipativeload (except for normal circuit losses) on the core solar cell 530 (aninductor temporarily coupled between the output of the core solar celland the circuit ground, which inductor is then switched to the circuitoutput to recover the magnetic energy temporarily stored in theinductor) and provides a DC output. In particular, while most switchingregulators typically control the switching to provide a regulatedvoltage output to a load, regardless of changes in the load, theSwitching Regulator 740 of FIG. 7D is controlling the voltage at itsinput (i.e., at the output of the core solar cell 530) so that thevoltage at the output of the core solar cell 530 swings between V_(min)and V_(max) at the desired frequency. This may be achieved by oneskilled in the art by control of the switching duty cycle, switchingfrequency or a combination of these or other parameters.

For the voltage swing to V_(max), note that V_(max) will be a highervoltage than the open circuit voltage of the core solar cell, so thatthe Voltage Control 730 that controls (V_(min), V_(max)) needs to havethe capability of pulling V_(max) to a voltage above the open circuitvoltage of the core solar cell. Thus in FIG. 7D, the Voltage Control 730that controls (V_(min), V_(max)) not only senses the voltage at the node720, but also has the capability of pulling the voltage at node 720 upbeyond the voltage it would tend to reach by itself. The impedance atthe node 720 for the pull-up is expected to be very high, as the coresolar cell 530 is still outputting current at the higher voltages. Inthat regard, V_(max) may be set at a voltage above which the powerrequired to raise the voltage on node 720 exceeds the power beingrecovered from the core solar cell 530, unless for some reason it isfound that some higher value of V_(max) is useful. It is also possiblethat V_(max) should be set at a voltage somewhat below the voltage forwhich the power required to raise the voltage on node 710 equals thepower being recovered from the core solar cell 530, as the overallconsideration is the total solar cell system efficiency, not theefficiency of some small part of the system. The small amount of powerfor the pull-up may be taken from the output to the Load, as shown inFIG. 7D.

In increasing the voltage at the node 720 toward V_(max), the switchingRegulator 740 would be relatively inactive, or perhaps totally inactive,with the ramp up to V_(max) being controlled primarily by the output ofthe core solar cell 530, the value of capacitor C₁ and the pull-up atnode 720, with the Switching Regulator 740 then becoming more active totransfer the charge from capacitor C₁ through the Switching Regulator740 to the Load and output capacitor C₂ faster than charge is added tothe capacitor C₁ by the core solar cell 530, causing the voltage oncapacitor C₁ to decrease to V_(min), after which the cycle repeats. Inthe limit, the voltage swing from V_(max) to V_(min) could be achievedin a single switching cycle of the Switching Regulator 740, withadjustments being made cycle to cycle to maintain the desired accuracyin V_(min). This would minimize the frequency requirements of theswitching regulator, with the wave shape of the voltage between V_(max)and V_(min) being controlled, at least in part, by the value ofcapacitor C₁. In FIG. 7D, an input to the Voltage Control 730 is shownto be f_(s)=1/T_(b) (see FIGS. 5B and 6). The actual switching frequencymay be f_(s) or may be higher, depending on the operation of the system.Also V_(max) and V_(min) could be programmable or self adapting toadjust for the characteristics of the light incident on the core solarcell—morning, mid-day, evening, sunny, overcast, artificial light, etc.

In FIG. 7D, the Switching Regulator 740 controls the voltage oncapacitor C₁ to achieve V_(min) and V_(max) at the desired frequency.Consequently the DC voltage on the Load is not regulated by thiscircuit, but would be effectively regulated by a circuit connectedthereto, such as an inverter, to convert the DC voltage on the Load to60 Hz for coupling to the power distribution system. Also the SwitchingRegulator 740 may be a step up, step down or step up/step downregulator, as desired. Such switching circuits are well known in theprior art and need not be further described herein.

Performance of the Alternating Bias Hot Carrier Solar Cell

In order to analyze the performance of the embodiment 700 of the hotcarrier solar cell of this invention, certain design parameters andimplementation details will have to be taken into account. The first ofsuch details is the approach used to implement the bias circuit 510 or520 and the mixer circuit 540 and how these circuits are to beintegrated with the core solar cell element 530 of the hot carrier solarcell 700. FIG. 8 illustrates an exemplary implementation of the hotcarrier solar cell 700 of this invention in which the bias circuit,either 510 or 520, and the mixer circuit 540 are implemented as anintegrated circuit (IC) device which is bonded to the backside of thesolar cell module. Since the hot carrier solar cell 700 includesadditional circuits other than the core solar cell 530, namely, eitherthe bias circuits 510 or 520 and the mixer circuit 540, which willconsume some of the power that would be generated by the cell, theefficiency of the hot carrier solar cell 700 must be assessed in termsof its resultant expected power-added efficiency PAE (PAE is a termadopted from RF circuit design according to which the PAE of an RFcircuit which achieves an input to output amplification gain is thedifference between the input and output powers of the circuit divided bythe supply power), which can be expressed as:PAE=η(IL_(M))−(P _(LO) +P _(M))/P _(L)  Eq. 1Where,

η is the solar power conversion efficiency that can be achieved by thehot carrier solar cell 700;

IL_(M) is the insertion loss of the output mixer 540 expressed asoutput-to-input power ratio;

P_(LO) is the power consumed by the bias circuit 510 or 520;

P_(M) is the power consumed by the mixer circuit 540; and

P_(L) is the radiant power of the solar radiation incident on the hotcarrier solar cell 700.

The values of IL_(M) and P_(M) are dependent on the design approach andthe power level handled by the output mixer 540 of the hot carrier solarcell 700. In order to quantitatively analyze the efficiency of the hotcarrier solar cell 700 as expressed by Eq. 1, it is assumed that thebias circuit 510 and output mixer circuit 540 illustrated in FIG. 7A aredesigned to drive a 100 cm² sub-cell area of the core solar 530. Itshould be noted that an arbitrary size of the hot carrier soar cell 700can be created using multiple such sub-cells as illustrated in FIG. 8,each with its own drive circuit, with their output combined to provide asingle output, either in AC or DC format. For the assumed sub-cell size,the radiant power of AM1.5 solar flux incident of the assumed sub-cellarea would be P_(L)=10 W. With such a level of solar incident lightradiant power and an expected solar power conversion efficiency for thehot carrier solar cell 700 of η=0.54, the expected power at the input ofthe mixer 540 would be in the range of 5.4 W. It should be noted for thepurpose of this performance analysis example, although the theoreticallypredicted efficiency of hot carrier solar cells can reach thethermodynamic limit of 68% under 1-sun, in order to allow for animplementation loss margin the expected value of the efficiency that thehot carrier solar cell 700 would achieve is conservatively selected tobe less than 80% of the predicted theoretical limit for the efficiencyof a hot carrier solar cell.

Based on these expected values, a 0.18 micron CMOS integrated circuitimplementation of the bias circuit 510 and the mixer circuit 540 isestimated to be able to conservatively achieve the following performanceparameters:

IL_(M)=0.95;

P_(LO)=108 mW,

P_(M)=270 mW; and

P_(L)=10 W.

When the above values are used in Eq. 1, the power-added efficiency(PAE) estimate for the hot carrier solar cell 700 used in our benchmarkdesign example is PAE=0.47, which is more than double the efficiency ofa typical conventional core solar cell 530 that would be used toimplement the hot carrier solar cell 700.

In taking into account the assumed efficiency of the hot carrier solarcell 700 of η=0.54, this benchmark design example implies that theadditional circuit used to generate the alternating bias and convert theoutput of the hot carrier solar cell 700, whether to AC or DC, wouldconsume approximately 12% of the cell output power. It should be notedthat conventional solar cells lose more than 25% of their yielded powerto the DC/AC inverter typically used at their output, yet the hotcarrier solar cell 700 of this invention would only lose less than halfthat percentage to its bias and mixer circuits, but would still enablemore than double the raw efficiency before the DC/AC inverter of theconventional core solar cell 530 used to implement the hot carrier solarcell 700 operating with a fixed bias. Meaning based on a comparison atthe AC output, the hot carrier solar cell 700 would likely achieve anoverall solar power conversion efficiency that will be almost 2.7× thatof a conventional solar cell with an AC/DC inverter at its output.

It should be noted that with the above estimated level of power-addedefficiency, a self-biasing scheme of the hot carrier solar cell 700would also be feasible. In such a self-biasing scheme, which isillustrated in FIG. 7C, the hot carrier solar cell 700 would not requireany additional power to be supplied in order to be initialized and wouldinitially operate in a non-alternating (fixed) bias mode with a smallportion of its generated energy being used to initialize its alternatingbias and mixer circuits. As illustrated in FIG. 7C, the power output ofthe hot carrier solar cell 700 will be used to provide supply power tobias circuit 510 and the mixer circuit 540, which are shown in FIG. 5Ccollectively enclosed in the dashed box 580 with their power supply line585 being provided from the solar cell output power 590. As soon as thebias circuit 510 and mixer circuit 540 reach steady state (which wouldbe less than a millisecond), the alternating bias mode of operation ofthe hot carrier solar cell 700 of this invention is expected to morethan double the output power yielded by the conventional core solar cell530 used to implement the hot carrier solar cell 700.

Cost Considerations

The alternating bias and mixer circuits described in the previousdiscussion are estimated to require ˜1 mm² of die area using 0.18 CMOStechnology. In order to reduce the packaging overhead cost that would beassociated with such a small die size, the bias and mixer circuits offour of the sub-cells illustrated in FIG. 8 can be readily combined ontoa single chip, which would be placed on the backside at the center ofeach four (Quad) sub-cells. Such a chip is estimated to have ˜4 mm² ofdie area using 0.18 micron CMOS technology with an estimated unpackageddie cost ˜$0.19/die. The packaged cost of this quad bias & mixer chip isconservatively estimated to be in the range of 5× the die cost, whichwould lead to an estimated cost per quad bias & mixer chip of ˜$1. Thisestimate is most likely on the conservative side, especially whenlow-cost packaging techniques such as glop-top are used to integrate thequad bias & mixer chip directly on the backside of the hot carrier solarcell 700 sub-cell as illustrated in FIG. 8. In taking into account theestimated output power yielded by the hot carrier solar cell 700 quadsub-cell based on the design example described earlier, the addition ofthe quad bias & mixer chip would result in a cost-per-watt offset ofδW_(p)˜$0.05/W—meaning that 25 of the quad bias & mixer chips would beused per square meter for an added cost of ˜$25/m² with an estimated netyielded output power from a Si-based hot carrier solar cell of ˜475W/m². In taking into account that the prevailing cost-per-watt for Sisolar cells is in the range from W_(p)˜3-5$/W, the estimated cost offsetof converting a conventional Si solar cell into the Si-based hot carriersolar cell 700 is estimated to be in the range from 1% to 1.6% of thecurrent value of the cost-per-watt W_(p) for Si solar cells, which forall practical purposes is negligible. In taking into account theincrease in the power output that can be realized by the hot carriersolar cell 700, the estimated cost-per-watt of a hot carrier solar cell700 that incorporates a Si solar cell as its core element 530 would bein the range (W_(p))˜1-1.7$/W, which reflects a per watt cost reductionof more than 3×. At this scale of cost-per-watt offset δW_(p) andefficiency increase, it is expected that the hot carrier solar cell 700of this invention that uses a thin-film type solar cell; for exampleCdTe, CIGS or thin-film Si, as a core solar cell 530 can achieve acost-per-watt W_(p) that is well below 1 $/W and possibly in the rangeof W_(p)˜0.3 $/W, which is well within the range set forth for the3^(rd) Generation solar cell.

Reverse Bias Hot Carrier Solar Cell

The embodiment 700 of the alternating bias hot carrier solar cell 500described in the previous discussion relies on cyclically lowering thephotovoltage output of the cell in order to instantaneously cause anincrease in the transport velocity of the photo-excited carriers. Analternative approach that would achieve a comparable effect, yet in adifferent way, would be to intermittently apply an external reverse biasat the cell contacts for a sufficiently short time interval. Theintermittent application of a short reverse bias pulse would introducean additional external electric field ε_(ext) across the cell contactthat will actually act to enforce the built-in field of the cell. Theresult would be that these reverse bias pulse intervals will cause thetransport velocity photo-excited carrier to instantaneously increaseabove the saturation velocity of the cell material, and depending on theamplitude of the applied reverse bias pulses, the photo-excited carrierstransport velocity could reach ballistic overshoot level. Thisalternative embodiment of the hot carrier solar cell 500 of thisinvention would use the parallel bias 520 to generate the reverse biaspulses that would achieve substantially shorter photo-excited carrierstransport time to the cell contacts. In addition, this alternativeembodiment of the hot carrier solar cell 500 of this invention wouldsimultaneously also use the series bias circuit 510, but in this case tosustain a high transport velocity, albeit lower than the transportvelocity during the duration of the applied reverse bias, and to alsoimplement the temporally selective extraction energy scheme describedearlier within the context of the embodiment 500 and 700. The primaryfeatures of this alternative embodiment of the hot carrier solar cell500 of this invention are that it allows the decoupling of the carriertransport and carrier extraction energy aspects of the hot carrier solarcell 500 of this invention. With the decoupling of these two aspects ofthe hot carrier solar cells of this invention it becomes possible tosustain a time continuous high value of carrier transport velocity byappropriately selecting the intermittence cycle of the applied reversebias pulse while being able to independently select the appropriatevalue for the cycle of variability of the cell photovoltage that enablesthe temporally energy selective scheme of the hot carrier solar cell 500and 700 to be able to timely extract the photo-excited carriers at thecell contacts before they cool down.

FIG. 9A illustrates the block diagram of an alternative embodiment ofthe hot carrier solar cell 500, which is now referred to as 900, thatuses a parallel bias circuit 920 to generate a stream of short durationreverse bias pulses across the contacts in addition to a series biascircuit 910, similar to the bias circuit 510, that causes the resistanceacross the cell 900 contacts, and thus the photovoltage output value ofthe hot carrier cell 900, to cyclically vary between a minimum valueV_(min) and a maximum value V_(max) in a substantially similar way as inthe embodiment 700 described earlier. The effect of the variable biasintroduced by the series bias circuit 910 would be to enable the hotcarrier solar cell 900 to extract the hot carriers at its contact at awide range of extraction energy E_(out) that will extend from slightlybelow the cell band-gap energy of its core solar cell element 530 up tothe maximum desired energy level that would be enabled by the maximumvalue of the photovoltage enabled by the bias circuit 920. In the caseof the hot carrier 900 of this invention, the waveform of thephotovoltage would no longer include the two sub intervals αT_(b) and(1−α)T_(b) that were explained earlier, but instead only the cycle timeT_(b) of the intermittent change of the cell photovoltage output willneed to be comparable or shorter in duration than the carrier coolingtime τ_(c).

FIG. 9B illustrates the waveform of the photovoltage of the hot carriersolar cell 900 which shows that the waveform can be a simple sinusoidalwaveform, for example, with its minimum value V_(min) and maximumV_(max) values selected as specified earlier. With the decoupling of thecarrier transport acceleration and the temporally selective energycarrier extraction aspects, the cycle time T_(b) of the intermittentchange of the cell photovoltage of the hot carrier solar cell 900 can bemade even shorter than that for the hot carrier solar cell 500 and 700.With T_(b)˜1 ns, the extraction energy E_(out) of the hot carrier solarcell 900 would change at a rate that would be even faster than thecarrier cooling rate in some solar cell material systems. In order torealize T_(b)˜1 ns, the frequency of the oscillator included in the biascircuit 910 will be f_(s)˜1 GHz.

Although the temporally varying photovoltage created by the series biascircuit 910 will still create the carrier transport acceleration effectdescribed earlier, a major part of the carrier transport accelerationeffect in the hot carrier solar cell 900 would be accomplished by theparallel bias circuit 920. The parallel bias circuit 920 will createvery short and periodic reverse bias pulses across the core solar cellelement 530. The waveform of the bias that will be generated by theparallel circuit 910 and applied across the core solar cell element 530is illustrated in FIG. 9B. The waveform illustrated in FIG. 9B isbasically a periodic stream of short pulses of reverse bias each with atime duration t_(p), a pulse repetition cycle T_(p) and an amplitudeV_(p). When each of the pulses generated by the parallel bias circuit920 is applied at the contact of the core solar cell 530, it will causean external electric field ε_(ext) that is in the same direction as thebuilt-in electric field ε_(bi) of the core solar cell 530, thus actuallyenhancing the effect of the built-in electric field ε_(bi) intransporting the electrons to the cell negative contact and the holes tothe cell positive contacts. The possible range of the values of V_(p)relative to the (I,V) characteristics of a conventional solar cell isillustrated in FIG. 4 with reference number 425.

During the time duration t_(p) of the reverse bias generated by theparallel bias circuit 920, the carriers will be transported toward thecontacts of the core solar cell 530 under the combined cooperativeeffect of the two electric fields ε_(bi) and ε_(ext) acting in the samedirection to transport the electrons toward the negative contact and theholes toward the positive contact of the hot carrier solar cell 900. Thefirst, and major, difference between the internal built-in electricfield ε_(bi) of the core solar cell 530 and the external field ε_(ext)caused by the applied reverse bias pulses generated by the parallel biascircuit 920 is that this external electric field ε_(ext) will extendcross the full thickness of the cell from contact to contact rather thanbeing present primarily within the core solar cell 530 depletion regionthickness. The second difference between the internal built-in electricfield ε_(bi) of the core solar cell 530 and the external field ε_(ext)caused by the applied reverse bias pulses generated by the parallel biascircuit 920 is that the intensity of the external electric field ε_(bi)can be set to the appropriate level needed to create the desired carrieracceleration effect. Furthermore, since the external electric fieldε_(ext) is applied periodically for a short time interval, the amount ofpower that would be consumed by the circuit that generates it, namely,the parallel bias circuit 920 would be very small.

During the reverse bias pulse duration t_(p) both the internal ε_(bi)and external ε_(ext) electric fields will be acting in the samedirection and will both contribute to the transport of the photo-excitedcarriers toward the contacts of the core solar cell 530. With theappropriate selection of the amplitude V_(p) of the applied reverse biaspulse, the combined strength of internal ε_(bi) and external ε_(ext)electric fields can be made to cause the carrier transport velocity toreach ballistic overshoot velocity, which is typically much greater than10⁷ cm/s, during the pulse duration t_(p) but will decay rapidly to thesaturation velocity level of ˜10⁷ cm/s. When reverse bias pulseamplitude V_(p), duration t_(p) and repetition cycle T_(p) are selectedappropriately (for example V_(p)˜1 V, T_(p)˜1 ns, and t_(p)˜0.1 T_(p))the carrier transport velocity across the core solar cell 530 can besustained continuously very close to the saturation velocity of ˜10⁷cm/s. That means that the photo-excited carriers transport toward thecontacts of the core solar cell 530 can be continuously sustained closeto 100 μm in 1 ns, which would allow photo-excited carriers generatedwithin a thin core solar cell 530 (such as CdTe, CIGS or Thin-Si) withcontact-to-contact thickness of ˜5 μm to be able to reach the cellcontacts within 25 ps, which is sufficiently shorter than the hotcarrier cooling time τ_(c) of most solar cell materials. These combinedfeatures of the hot carrier solar cell 900 would also make it possibleto apply in a conventional Si solar cell with typical contact-to-contactthickness ˜300 μm with the realized carrier transport time in this caseof ˜1.5 ns, which is also sufficiently shorter than the hot carriercooling time τ_(c) of the Si material.

With collective bias generated by the serial and parallel bias circuits910 and 920, respectively, the hot carrier solar cell 900 can transportthe photo-excited carriers to the contacts of the core solar cell 530well before the carriers cool down and have these carriers extracted ata temporally varying selective extraction energy also before they cooldown at the cell contacts. The serial bias circuit 510 block diagramused in the hot carrier solar cell 900 is substantially similar to thatof the serial bias circuit 510 referenced in FIG. 5C. FIG. 9Dillustrates a typical block diagram of the parallel bias circuit 520that can be used in conjunction with the embodiment 900 of the hotcarrier solar cell 500 of this invention. The type of circuitillustrated in FIG. 9D is similar to those typically used in ultrawideband wireless applications and can be designed to generate pulses offewer than picoseconds in duration with a repetition interval smallerthan one nanosecond. Such design parameters can readily be applied togenerate the reverse bias waveform illustrated in FIG. 9C. A personskilled in the art would know that there are many alternative circuitdesigns other than that illustrated in FIG. 9D that can be used togenerate the reverse bias waveform illustrated in FIG. 9C with acomparable end result.

As illustrated in FIG. 9A, similar to the hot carrier solar cell 500,the ultimate output of the hot carrier solar cell 900 would be downconverted either to DC or AC format by mixing the output of the serialbias circuit 910 with the cell output across the load resistor R_(L)except that in the case of the embodiment 900, the low pass filter 950will have to be added to remove the applied wideband spectrum of thereverse bias from the output of the cell 900 prior to the downconversion accomplished by a mixer circuit 540. Although FIG. 9Aillustrates the hot carrier solar cell 900 configuration that providesAC output, a configuration of the hot carrier solar cell 900 can readilybe realized to provide DC output by eliminating the 60-Hz oscillator 745and the mixer 750 from the block diagram of FIG. 9A.

In reference to the earlier discussion on the loss mechanisms in solarcells, the hot carrier solar cells 500, 700 and 900 of this inventionachieve yielded net efficiency increase by circumventing two major lossmechanisms; namely, loss mechanism

, hot carrier cooling, and loss mechanism

, contact extraction efficiency. The discussion in the following sectionis intended to show that when the alternating bias scheme of thisinvention is implemented in conjunction with core solar cellsincorporating quantum confinement structures, such as QWs or QDs, theresultant hot carrier solar cell would be able to achieve a still higheryielded net efficiency increase by circumventing another one of themajor loss mechanisms, namely, loss mechanism

, the loss of photons with E_(p)<E_(g), plus loss mechanism

, the loss of photo-excited carriers due to radiative recombination. Aswill be explained in a later discussion, the hot carrier solar cells500, 700 and 900 that use a core solar cell 530 that incorporates bothoptical and quantum confinement structures are likely to achieve yieldednet efficiency that would surpass that achieved by multi-junction solarcells, thus indirectly avoiding loss mechanism

by avoiding the need for monolithic multi-junction staking and thelattice mismatch issue associated with it altogether—the main instigatorof loss mechanism

.

Extended Coverage Alternating Bias Hot Carrier Solar Cells

Having described in the previous discussion multiple embodiments of thealternating bias hot carrier solar cell of this invention that use p-njunction solar cells such as Si, GaAs, CdTe and CIGS as its core solarcell 530, the discussion in this section is aimed toward extending thecapability the alternating bias solar cell of the invention to alsoharvest the energy of incident solar photons with energies E_(p) belowthe cell band-gap E_(g). The path to achieve this objective is to applythe alternating bias solar cell of this invention in conjunction withIII-V material solar cells incorporating quantum confinement structuressuch as QWs and QDs. This is an attractive application of thealternating bias solar cells of this invention because the versatilematerial band-gap options of III-V alloys plus their direct band-gap andhigh carrier mobility when combined with the alternating bias scheme ofthis invention could lead to a single junction solar cell that wouldhave an extended coverage of the solar spectrum and offer extremely highyielded net efficiency. Although the following discussion will belimited to MQW based solar cells, other than the effects of the extradimension of quantum confinement, the underlying concepts behind a QDbased alternating bias hot carrier solar cell are substantially similar.

Photovoltaic (PV) solar cells that use quantum confinement structuressuch as QWs and QDs have been extensively studied, but even though theyare predicted to achieve efficiency enhancement due to extending thesolar photon absorption below the intrinsic cell band-gap (“Quantum WellSolar Cells”, K. W. J. Barnham et al, Physica E14 (2002) 27-36), theyare yet to gain wide use in comparison to bulk material solar cells,mostly because of the imbalance between their predicated efficiencyenhancement and the increase in the cell cost. This imbalance is causedby the fact that quantum confinement based solar cells band-gapextension would mostly be at the lower energy side of the band-gap, thusonly increasing the cell photon absorption capability toward the longerwavelength. In addition, the width of the achieved cell band-gapextension would be highly dependent on the material system used and theband-gap structure of the incorporated quantum structure. However, asdiscussed earlier, the incorporation of quantum structures within thesolar cell would offer the ability to slow down the cooling (extendingthe cooling time τ_(c)) of hot carriers in III-V material alloy systems.The resultant prolonged carrier cooling time τ_(c) in solar cells thatincorporate quantum structures makes it more feasible to apply thealternating bias scheme of this invention to III-V material based solarcells since the carrier cooling time in such materials is typicallyshorter than that in Si, CdTe or CIGS material systems. The benefits ofapplying the alternating bias hot carrier extraction scheme of thisinvention of enabling the extraction of photo-excited carries withenergy extending beyond the cell material band-gap E_(g) will equallyapply to III-V material based solar cells that incorporate quantumstructures. The combined effect of the incorporated quantum structuresextending the photo-excited carrier energy extraction below the cellmaterial band-gap E_(g) and the alternating bias of this inventionextending the photo-excited carrier energy extraction above the cellmaterial band-gap E_(g) would result in a solar cell that would have anextended coverage that could possibly span a substantial portion of thesolar spectrum. For example when the alternating bias scheme of thisinvention is applied in conjunction with a GaAs based solar cell thatincorporates quantum confinement structures such as QWs or QDs, thephoto-excited carrier extraction of the resultant hot carrier solar cellof this invention can be made to extend well below and above theband-gap energy value of E_(g)=1.42 eV of GaAs.

The alternating bias scheme of this invention, described in the previousdiscussion, can be applied in conjunction with a MQW-based solar cell invery much the same way as the embodiments 500, 700, or 900 with the coresolar cell 530 being a III-V material based solar cell that incorporatesquantum confinement structures such as QWs or QDs. The energy bandstructure of such a core solar cell 540 is illustrated in FIG. 10A,which shows MQWs being incorporated within the intrinsic region of ap-i-n junction solar core solar cell (“Quantum Well Solar Cells”, K. W.J. Barnham et al, Physica E14 (2002) 27-36). The incorporated MQWs wouldbe designed to extend the cell energy extraction capabilities below thecell material band-gap energy E_(g). This would be accomplished bytapering (grading) the band energy of the MQWs to provide a widecoverage of the energy bands below the cell band-gap E_(g). What ismeant by graded MQWs is illustrated in FIG. 10A, which shows theband-gap associated with each of the incorporated QWs having a differentvalue spanning from E_(a) to E_(b), both of which are below the cellmaterial band-gap E_(g). The range of energy levels from E_(a) to E_(b)can be viewed as an extension of the photon energy absorption capabilityof the cell below the intrinsic band-gap energy E_(g). Meaning it wouldenable photo-excitation of carriers by incident solar photons withenergies within the range from E_(a) to E_(b), below the cell band-gapenergy E_(g). In effect, this approach substantially overcomes the cellefficiency loss mechanism

explained earlier, as it would allow the cell to convert the energy ofincident solar photons at or below the cell band-gap E_(g). A detailedexplanation of this effect is provided in the discussion below.

An illustration of the energy band structure of the MQW-basedalternating bias hot carrier cell under illumination is shown in FIG.10B. Under illumination, the band structure of the MQW-based alternatingbias hot carrier cell can be described by at least three quasi-Fermilevels (QFLs) (“Detailed Balance Efficiency Limits with Quasi-FermiLevel Variations”, S. P. Bremner, R. Corkish and C. B. Honsberg, IEEETrans. Electron Devices, vol. 46, No. 10, October 1999; A. Luque and A.Marti, Ultra-high efficiency solar cells: the path for mass penetrationof solar electricity, Electronics Letter, vol. 44, No. 16, July 2008):

-   -   QFL_(V), which describes the population of holes in the cell        valance band (VB);    -   QFL_(I), which describes the population of the electrons and        holes (carrier pairs) in the intermediate band (IB) formed by        the graded MQWs; and    -   QFL_(C), which describes the population of electrons in the        conduction band (CB) of the cell.

Multiple solar photon absorption with energies at or below the cellmaterial band-gap E_(g), designated in FIG. 10A as P₁, P₂ and P₃,respectively, will enable the following multiple carrier transitionsbetween the multiple QFL separation created, illustrated in FIG. 10B, bythe graded MQWs:

VB

CB VB

IB IB

CB

When the alternating bias is used with the hot carrier solar cells 500,700 or 900 that incorporate a quantum confinement based core solar cell530, the carriers produced by the “extra” low energy photons P₂ and P₃absorbed by the graded MQWs illustrated in FIG. 10A will be extractedand contribute “extra” current. This extra current comes from theextended absorption of photons at lower energy (relative to the cellband-gap) and also from the increased carrier population due to theeffect of the quantum confinement in the graded MQWs (“Quantum WellSolar Cells”, K. W. J. Barnham et al, Physica E14 (2002) 27-36). Theprimary difference in this case will be that the temporally varyingphotovoltage of the hot carrier solar cells 500, 700 and 900 of thisinvention will also be extended in its V_(min) to provide the lowerextraction energy at the contact needed to extract the photo-excitedcarriers having energy below that of the core cell 530 material band-gapenergy E_(g). An added advantage of this embodiment of the alternatingbias hot carrier solar cell of this invention is that the alternation ofthe solar cell photovoltage to the lower value needed to extract thephoto-excited carrier with energy below the cell material band-gap E_(g)will also allow the cell to periodically operate at a lower photovoltagevalue, which in turn enables a periodic increase in the carriertransport electric field value that enhances the carrier transportcapability of the cell. This means that during the time interval whenthe cell photovoltage is lowered to enhance the carrier transportcapability of the hot carrier solar cells 500, 700 and 900 of thisinvention, the lowered value of photovoltage is leveraged to provide thelower value of extraction energy at the cell contacts needed to extractthe photo-excited carriers with energy below the cell material band-gapenergy enabled by the quantum structures incorporated within the coresolar cell of the hot carrier solar cell of this invention.

Optical Confinement Alternating Bias Hot Carrier Solar Cell

As explained earlier, the contact-to-contact thickness of the core solarcell 530 used in the hot carrier solar cell embodiments 500, 700 and 900significantly affects the carrier transport time, and consequently theperformance of the alternating bias hot carrier solar cells of thisinvention. For example, even though the carrier lifetime characteristicsin Si are typically much longer than III-V materials, as explainedearlier, for silicon based core solar cell 530, a carrier transport timethat ranges from ˜1.5 ns to ˜15 ns can be achieved by the hot carriersolar cells 900 and 500. These values are the high end of the carriercooling phase of the carrier lifetime illustrated in FIG. 2. As aresult, the benefits of the alternating bias hot carrier extractionscheme of this invention may not be fully realized in solar cells with alarge contact-to-contact thickness, such as conventional Si solar cells.However, for cost reduction reasons, there is an intensive ongoingactivity aimed at reducing the Si-based solar cell thickness; a trendthat will also allow the benefits of the alternating bias hot carrierextraction scheme of this invention to be fully realizable in suchcells. For example, the texturing of the cell top surface plus theplacement of a reflective surface at the back side of a Si-based solarcell will cause multiple reflections of the light that enters the cellabsorber, which will in turn allow the solar photons incident on thecell to be absorbed with a much thinner cell absorber. This simple lighttrapping means can cause a 20 μm thick Si solar cell to have even betterlight absorptance than a 400 μm Si solar cell (“Physics of Solar Cells”,P. Würfel, pp. 173-177).

A light confinement solar cell structure that leverages light trappingaspects similar to those referenced earlier plus the buried contacts(“The Physics of Solar Cells”, J. Nelson, pp. 188-191) is illustrated inFIG. 11A. As illustrated in FIG. 11A, such a cell structure wouldincorporate reflective vertical sidewalls that divide the overall cellinto multiple sub-cells each typically would be of width L_(c) on theorder of a few tens of microns. These sidewalls would serve multiplepurposes. First, the sidewalls would be used to create multiplicity ofoptical micro cavities across the entire cell. These micro cavitieswould provide optical confinement of the incident solar photons that didnot cause photo-excitation of carriers during their first pass, as wellas photons generated within the cell structure due toradiative-recombination of photo-excited carriers. Second, asillustrated in FIG. 11A, the sidewalls would also serve as buriedcontact rails that would be interconnected to a micro mesh at the topsurface of the cell. As illustrated in FIG. 11A, the micro mesh wouldhave orthogonal pitch on the order of a few microns and would span theentire topside surface of the sub-cell in order to provide uniformdistribution of applied bias as well as uniform contact for theextraction of the cell power output without significant blocking of thelight incident of the cell top surface.

The optical confinement achieved by the reflective micro cavitysub-cells sidewalls, backside and texture topside of the micro cavityillustrated in FIG. 11A would result in the light that enters thesub-cell, reflecting as many as 4-6 times before decaying due to theabsorptance of the micro cavity sub-cell sidewalls and backside. Whenthe distance between micro cavity sub-cell sidewalls is L_(c)˜50 μm, thecreated light confinement capability of the micro cavity sub-cellstructure illustrated in FIG. 11A would make it possible to achievecontact-to-contact thickness of ˜5 μm for a Si-based solar cell thatincorporates the optical confinement micro cavity illustrated in FIG.11A. As explained earlier, this contact-to-contact thickness will enablea carrier extraction time of ˜0.25 ns, which is well within the expectedcarrier cooling time for silicon cells. When the light confining cellstructure illustrated in FIG. 11A is used as the structure of the corecell 530 in the configurations 500, 700 and 900 of the hot carrier solarcells of this invention, it would allow these cells to operate atshorter carrier transport times. In addition it would also facilitatecarrier photo-excitation by photons that would be generated by radiativerecombination of photo-excited carriers—in a typical solar cell, thesephotons are lost due to efficiency loss mechanism

explained earlier because they are not confined within such cellstructures.

It is worth mentioning that the combination of the optical confinementprovided by the cell structure incorporating the optical confinementmicro cavity of FIG. 11A and the carrier quantum confinement provided bythe cell material structure of FIG. 10A, when used collectively asillustrated in FIG. 11B within the context of the core solar cell 530 ofthe alternating bias solar cells 500, 700 and 900, would contribute toincreasing the probabilities of carrier excitation by photons that didnot cause carrier excitation in the first pass through. The possibilitythat the photons generated by the radiative recombination would causesubsequent excitation of carriers(s) can be greatly enhanced by theprolonged lifetime of the generated photon due to the opticalconfinement of the sub-cell micro cavity as well as the carrierconfinement effect of the MQWs. Thus, the combination of photons andcarrier confinement aspects of FIG. 11B would in effect enhance thechances that one solar photon would cause the excitation of multiplecarriers—an effect that would circumvent one of the key assumptions ofthe efficiency limit established by the SQ-Model discussed earlier;namely, that a solar incident photon is assumed to produce only oneelectron-hole pair.

In reference to the earlier discussion on the loss mechanisms in solarcells, the hot carrier solar cells of this invention would be able toachieve a yielded net efficiency increase by circumventing lossmechanisms:

the loss of photons with E_(p)<E_(g), loss mechanism,

the loss due to hot carrier relaxation,

the loss of photo-excited carriers due to radiative recombination, and

the loss due to contact extraction efficiency. Furthermore, the hotcarrier solar cells of this invention would altogether avoid lossmechanism

because it would be able to achieve efficiency comparable to that of amonolithic multi-junction cell with a single junction cell structure,thus avoiding the lattice matching issue that invokes that lossmechanism.

Based on the above discussion, the alternating bias hot carrier solarcells of this invention possibly incorporating quantum confinementstructures such as QWs or QDs and photonic micro cavity sub-cells wouldbe able to:

1. Convert into power the energy of incident solar photons havingenergies at, above and below the cell material band-gap E_(g);

2. Harness the energy associated with internal photoemission caused byradiative re-combination of photo-excited carriers by re-cycling theemitted photons which otherwise will be lost;

3. Achieve multiple carrier-pair excitation by a single incident solarphoton;

4. Enable the extraction of photo-excited carriers from cell structurethat incorporate multiple QFLs separation; and

5. Operate in an alternating output mode that eliminates the AC/DCinverter loss;

Candidate Material Systems

As explained earlier, the alternating bias hot carrier solar cells ofthis invention can be implemented in conjunction with conventional bulkmaterial solar cell materials, such as conventional Si, CdTe, CIGS, aswell as III-V materials such as bulk GaAs and solar cells incorporatingquantum confinement such as QWs and QDs. FIG. 12 (adapted from “ThirdGeneration Photovoltaics”, Gregory F. Brown and Junqiao Wu, Laser &Photon Rev., 1-12 (2009), published online: 2 Feb. 2009) shows theband-gap energy of several candidate solar cell material systems thatcan be used in conjunction with the alternating bias hot carrier solarcells of this invention in reference to the energy distribution of thesolar spectrum. As shown in FIG. 12, today's most efficient singlejunction solar cells, including those based on Si, InP and GaAs, haveband-gap energies between 1.1 eV and 1.4 eV and can typically convertinto energy only a relatively narrow band of the solar photons acrossthe solar energy spectrum. This narrow coverage of the solar energyspectrum is fundamentally what is limiting today's single junctionphotovoltaic solar cell efficiency below 30% under 1-sun. In practice,the yielded net efficiency is even lower due to implementation losses, alarge portion of which is the 25% DC-to-AC inverter loss, which makesthe yielded net efficiency of a typical single junction solar cellmodule become well below 20%. In practice, the best efficiency singlejunction Si-cell achieves less than 24% efficiency, not counting theDC-to-AC inverter loss, which when taken into account would result in18% yielded net efficiency from the cell. In comparison, the alternatingbias hot carrier solar cells of this invention could have a solar photonenergy coverage that would extend from 0.65 eV to 3.15 eV, thus coveringa large portion of the solar spectrum and yielding a net efficiencylevel that is multifold higher than 18%.

The solar energy spectrum coverage of two candidates alternating biashot carrier solar cells is illustrated in FIG. 12. The first candidatebeing Si-based, designated in FIG. 12 as Si-ABC, uses either aconventional Si solar cell or the thin-Si solar cell described in FIG.11A that incorporates optical confinement micro cavities. The secondcandidate is based on using a In_(x)Ga_(1-x)N material system thatincorporates the graded MQWs, designated in FIG. 12 as InGaN MQW-ABC.With the alternating bias hot carrier extraction scheme of thisinvention, it would be possible to extend the Si-based solar cellspectrum coverage to have it span from the Si band-gap energy of 1.1 eVto almost 3 eV, thus enabling overall power conversion efficiency thatcould be more than 2.5× of a conventional Si-cell. With the typicalyielded power retail cost of 0.35 $/kWh of today's high efficiencySi-based solar cells (“Solar Photovoltaics: Expanding ElectricGeneration Options”, Electric Power Research Institute (EPRL), December2007) and in taking into account cost increase due to the componentsadded to implement the alternating bias scheme of this invention, theefficiency increase that can be achieved by a Si-based alternation biassolar cell would yield a retail cost of solar power of less than0.15$/kWh, which is less than half the retail cost of power achieved bytoday's Si-cell and is well within the range of today's conventionalpower retail cost.

An indium gallium nitride (In_(x)Ga_(1-x)N) material system has aband-gap energy that spans from 0.65 eV to 3.4 eV, making it an almostperfect match to the solar spectrum. The full potential solar coverageof this material system can be achieved by a MQW-based In_(x)Ga_(1-x)Nalternating bias hot carrier solar cell described earlier. The gradedMQW of FIG. 10A can be realized in conjunction with using(In_(x)Ga_(1-x)N) material by varying the value of the indiumincorporation “x” across the multiple quantum wells from a low to a highvalue to create a multiplicity of quantum wells having a band-gap thatspans across the band-gap of gallium nitride (GaN). With the properdesign of the intermediate band of a MQW-based In_(x)Ga_(1-x)N material,the In_(x)Ga_(1-x)N material can be made to realize a band-gap thatextends from almost 0.65 eV to 3.4 eV. This type of material system,when used in conjunction with the alternating bias scheme of thisinvention, would be able to achieve almost full coverage of the solarspectrum, making it possible to achieve ultra high efficiency from asingle junction solar cell, especially when used in conjunction with asolar concentrator. As explained earlier, with such a cell output powerbeing already an alternating current, most of the achieved efficiencycan be harnessed to provide the potential of achieving higher than 70%yielded net efficiency after taking into account other implementationlosses. At this level of yielded net efficiency and an estimated costW_(p) of 2.25 $/W, the In_(x)Ga_(1-x)N MQW alternating bias hot carriersolar cell operating in conjunction with a 100× solar concentrator wouldhave the potential of achieving a retail cost of solar power of lessthan 0.10$/kWh—which is less than a third of the retail cost of powerachieved by today's most efficient conventional solar cells and is alsowell within the range of today's conventional power retail cost.

As explained above, the two examples of applications of the alternatingbias scheme in bulk Si-cell and MQW-based III-V solar cell described inthis disclosure show a predicted multifold reduction in the retail costof the power generated by the cell, which indicates that the alternatingcurrent hot carrier solar cell of this invention could lead toattainment of the 3^(rd) Generation (3G) solar cells cost goals.

Performance Comparison

Table 1 is a tabulation of the achieved efficiency of the most currentlyused solar cells together with the predicted yielded net efficiency (orpower added efficiency, PAE) of the two example applications of thealternating bias hot carrier solar cell discussed earlier; namely, theSi-based cell operating under 1-sun and the In_(x)Ga_(1-x)N MQW basedcell operating with a 100× solar concentrator (100-sun). In order to putthe comparison of Table 1 into perspective, it should be noted that thelisted achieved efficiency of the current solar cells does not reflectthe estimated 25% loss caused by the DC-to-AC converter needed at theiroutput. On the other hand, since the alternating bias hot carrier cellspower output is AC, the predicted efficiency performance of the twoalternating current cells listed in Table 1 is the yielded netefficiency at the system level after accounting for possibleimplementation losses. Therefore, for meaningful one-to-one comparisonthe efficiency performance values of the current solar cell should bedecremented by 25%.

Table 1 highlights the theme carried throughout this disclosure thatsolar cells implemented using the described alternating bias scheme ofthis invention could achieve multifold increases in efficienciesachieved by current single junction cells. Furthermore, Table 1 alsohighlights that, depending on the selected material system, the quantumconfinement structure based, either Qws or QDs, has the potential ofachieving a yielded net efficiency that is comparable or possibly higherthan multi-junction solar cells. When realized, the cost/efficiencybenefits of achieving this level of yielded net efficiency could verypossibly launch the solar cell industry in its way toward achieving theset 3G objectives.

TABLE 1 Comparison of the efficiency predictions of alternating currenthot carrier cells of this invention and conventional solar cells.Approximate Theoretical Experimental & Predicted Efficiency LimitPerformance Thermodynamic 87% — (max concentration) Single-junction (10033% 65+%  MQW-based III-V Alternating suns) Bias Hot Carrier cell⁽²⁾Thermodynamic (1 sun) 68% — Six-junction 59% — Single-junction (1 sun)33% 45+%  Alternating Bias Hot Carrier cell⁽²⁾ Triple-junction 64% 44%III-V alloys, monolithic concentrator stack⁽¹⁾ Double-junction 56% 30%III-V alloys, monolithic concentrator stack⁽¹⁾ Triple-junction (1 sun)51% 15% Thin-film amorphous silicon alloys⁽¹⁾ Double-junction (1 sun)45% 12% Thin-film amorphous silicon alloys⁽¹⁾ Shockley-Queisser 33% 24%Crystalline silicon Single-junction (1 sun) 20% Thin multi-crystallinesilicon 12% Dye-sensitized cell  6% Organic cell ⁽¹⁾DC/AC Inverter LossNot Included ⁽²⁾No DC/AC Inverter Needed

CONCLUSION

This disclosure describes novel design approaches for achievingextremely high efficiency in solar cells. First, a novel alternatingbias scheme was described that enhances the photovoltaic powerextraction capability above the cell band-gap by enabling the extractionof hot carriers. When applied in conjunction with bulk material singlejunction solar cells, the described alternating bias hot carrier cellshas the potential of more than doubling its core cell yielded netefficiency. Second, when the alternating bias scheme is applied inconjunction with quantum wells (QWs) or quantum dots (QDs) based solarcells, the alternating bias hot carrier solar cells of this inventionhave the potential of extending their core solar cell power extractioncoverage across the entire solar spectrum, thus enabling the attainmentof an unprecedented level of solar power extraction efficiency. Third,when the alternating bias scheme is applied in conjunction with coresolar cells incorporating both quantum and photonic confinement, theresultant solar cells can potentially circumvent most all of the lossmechanisms that limit the efficiency of today's solar cells. This isachieved by combining the hot carrier extraction capability of thedescribed alternating bias scheme with a novel cell design thatincorporates graded MQWs to extend the cell photovoltaic powerextraction capability below the cell band-gap and sub-cell photonicconfinement micro cavities to harness the carriers radiativerecombination and to enable the generation of multiple carriers persingle absorbed photon, thus further enhancing the cell efficiency.

Thus the present invention has a number of aspects, which aspects may bepracticed alone or in various combinations or sub-combinations, asdesired. While preferred embodiments of the present invention have beendisclosed and described herein for purposes of illustration and not forpurposes of limitation, it will be understood by those skilled in theart that various changes in form and detail may be made therein withoutdeparting from the spirit and scope of the invention.

In the forgoing detailed description, the present invention has beendescribed with reference to specific embodiments thereof. It will,however, be evident that various modifications and changes can be madethereto without departing from the broader spirit and scope of theinvention. The design details and drawings are, accordingly, to beregarded in an illustrative rather than a restrictive sense. Skilledpersons will recognize that portions of this invention may beimplemented differently than the implementation described above for thepreferred embodiment. For example, skilled persons will appreciate thatthe serial and parallel bias circuits of the alternating bias hotcarrier solar cells of this invention can be implemented with numerousvariations and that the specific design parameters of these biascircuits can substantially vary the characteristics of the alternatingbias and consequently the performance of the resultant solar cell.Skilled persons will further recognize that many changes may be made tothe details of the aforementioned embodiments of this invention withoutdeparting from the underlying principles and teachings thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

What is claimed is:
 1. A method of operating a solar cell having ajunction with a p-doped side and an n-doped side, and first and secondcontacts, the method comprising: causing the photovoltage across thefirst and second contacts to periodically alternate between a minimumand a maximum voltage value of a same polarity during operation of thesolar cell; the period of alternation of the photovoltage between theminimum and maximum voltage values being shorter than the hot carriercooling time for the solar cell to extract from the solar cell, duringthe alternation of the photovoltage, photo-excited carriers across arange of energy levels, including photo-excited hot carriers.
 2. Themethod of claim 1 wherein the solar cell is a bulk material solar cell.3. The method of claim 1 wherein the solar cell incorporates quantumconfinement.
 4. The method of claim 1 wherein: the minimum photovoltagevalue is a photovoltage value at which the solar cell built-in potentialis sufficiently high to cause acceleration of the photo-excited carrierswithin the solar cell toward first and second contacts of the solar cellwith a transport velocity approaching or reaching its maximum value; andthe maximum photovoltage value being substantially equal to the maximumvalue of the electrochemical potential of the photo-excited carriersgenerated within the solar cell.
 5. The method of claim 4 wherein asub-period during which the solar cell photovoltage value approaches orreaches the minimum photovoltage value is selected to be short enough tomaintain the average photovoltage achieved by the solar cell at or nearthe highest possible value and/or is selected to be long enough tosustain an average carrier transport velocity to transport substantiallyall of the photo-excited carriers within the solar cell to the first andsecond contacts of the solar cell within the hot carrier cooling time.6. The method of claim 1, further including at least one time interval,shorter than the period of alternation, during which the solar cellphotovoltage reaches an opposite polarity value of a duration and periodof repetition to sustain an average carrier transport velocitysufficient to transport substantially all of the photo-excited carrierswithin the solar cell to the first and second contacts within the hotcarriers cooling time.
 7. The method of claim 6 wherein a sub-periodduring which the solar cell photovoltage value approaches or reaches theminimum photovoltage value is selected to be long enough to sustain anaverage carrier transport velocity to transport substantially all of thephoto-excited carriers within the solar cell to the first and secondcontacts of the solar cell within the hot carrier cooling time, thesub-period during which the solar cell photovoltage value approaches orreaches the minimum photovoltage value is selected to be short enough tokeep the average photovoltage achieved by the solar cell at the highestpossible value, and the period of periodically alternating betweenminimum and maximum photovoltage values and the ratio of the duration ofthe sub-period to the alternation period being selected responsive tothe band-gap, carrier mobility and crystal lattice characteristics ofthe solar cell, allowing the extraction energy separation between thesolar cell contacts to temporally sweep through a range of extractionenergies that substantially matches the profile of the electrochemicalpotential of the photo-excited carriers within the solar cell, thusallowing a single junction solar cell to have the energy extractionefficiency benefits of a multi junction solar cell.
 8. The method ofclaim 6 wherein a sub-period during which the solar cell photovoltagevalue approaches or reaches the minimum photovoltage value is selectedto be long enough to sustain an average carrier transport velocity totransport substantially all of the photo-excited carriers within thesolar cell to the first and second contacts of the solar cell within thehot carrier cooling time, the sub-period during which the solar cellphotovoltage value approaches or reaches the minimum photovoltage valueis selected to be short enough to keep the average photovoltage achievedby the solar cell at the highest possible value, and the period ofperiodically alternating between minimum and maximum photovoltage valuesand the ratio of the duration of the sub-period to the alternationperiod being selected responsive to the band-gap, carrier mobility andcrystal lattice characteristics of the solar cell, thereby providing theextraction energy separation between the solar cell contacts totemporally sweep through a wide range of extraction energies at a ratethat is comparable to or faster than the hot carrier cooling rate,allowing the photo-excited carriers that reach the solar cell contactsto be transferred to a solar cell load through a temporally discretenarrow extraction energy band at each contact and with instantaneousenergy separation between the contacts that substantially equals theenergy separation between the photo-excited carriers within the solarcell.
 9. The method of claim 6 wherein the solar cell materials areselected from the group consisting of silicon (Si), gallium arsenide(GaAs), cadmium telluride (CdTe), copper indium diselenide (CIS), copperindium gallium diselenide (CIGS), and alloys of III-V materials.
 10. Themethod of claim 9 wherein the alternation of the photovoltage values ofthe solar cell between the minimum and maximum photovoltage valuesextracts photo-excited carriers within the solar cell over a range ofextraction energies that substantially matches the energy profile of thephoto-excited carriers generated within the solar cell that spans fromthe band-gap energy of the solar cell materials to an energy that issubstantially equal to the maximum value of the electrochemicalpotential of the hot carriers to be extracted from the solar cell. 11.The method of claim 9 wherein the solar cell comprises either quantumconfinement structures or optical confinement structures or both. 12.The method of claim 11 wherein the alternation of the photovoltagevalues of the solar cell between the minimum and maximum photovoltagevalues extracts carriers within the solar cell that were photo-excitedby solar photons having energy that extends over a range of energiesextending from the band-gap energy of the solar cell materials to anenergy that is substantially equal to the maximum value of theelectrochemical potential of the hot carriers to be extracted from thesolar cell.
 13. The method of claim 11 wherein the alternation of thephotovoltage values of the solar cell cause the extraction of carrierswithin the solar cell photo-excited by solar photons having energy thatextend over a wide range of energies from substantially below theband-gap energy of the solar cell materials to an energy that issubstantially equal to the maximum value of the electrochemicalpotential of the hot carriers to be extracted from the solar cell. 14.The method of claim 11 wherein the solar cell comprises quantumconfinement structures having multiple quantum wells wherein theband-gap of the multiple quantum wells is graded to provide a range ofdifferent band-gap values for the quantum wells, with the range ofdifferent band-gap values being below the solar cell material band-gapvalue.